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. 2020 Sep 4;15(9):e0238598. doi: 10.1371/journal.pone.0238598

Radiographic evaluation of patellar tendon length following corrective surgical procedures for medial patellar luxation in dogs

Kevin de Moya 1, Stanley Kim 2,*
Editor: Silvia Sabattini3
PMCID: PMC7473547  PMID: 32886725

Abstract

Objective

To quantify changes in the patellar tendon length following surgical correction of medial patellar luxation in dogs and evaluate potential risk factors associated with patellar tendon elongation.

Study design

Retrospective case series (n = 50).

Methods

Dogs that underwent surgery for medial patellar luxation correction and had 2–3 months follow up were included. Digital radiographs were utilized to quantify the patellar tendon length to patellar length ratio at various follow-up points. Odds ratio comparisons between potential risk factors associated with changes in patellar tendon length were performed.

Results

Post-operative patellar tendon lengthening of ≥ 5% was observed in 20% of stifles and post-operative patellar tendon shortening of ≥ 5% was observed in 22% of stifles at the 2–3 month follow up period. The risk factors including age, body weight, trochleoplasty and grade of medial patellar luxation were not significantly associated with risk of patellar tendon elongation. Patellar tendon lengthening was not associated with recurrence of luxation.

Conclusion

Patellar tendon lengthening and shortening can be observed in dogs following common medial patellar luxation corrective procedures in the short term follow up period. Patellar tendon lengthening does not appear to be associated with age, weight, trochleoplasty, grade of luxation, or risk of luxation recurrence.

Introduction

Medial patellar luxation (MPL) is one of the most commonly diagnosed orthopedic diseases affecting the canine stifle [1]. Skeletal abnormalities including quadriceps displacement, distal femoral varus, genu varum, proximal tibial varus, shallow trochlear groove, coxa vara, coxa valga, hypoplastic trochlear ridges, patella alta and tibial torsion have all been associated with MPL [2–3]. Additionally, MPL has been associated with patella alta and a relatively longer patellar tendon length in medium to giant breed dogs [4]. Surgical intervention aims to properly align the quadriceps mechanism, restore normal patellar tracking within the trochlear groove, and address significant underlying skeletal abnormalities [5]. Failure to identify underlying contributing factors may lead to sub-optimal outcomes, including luxation recurrence and progression of osteoarthritis [6, 7].

Post-operative complications are associated with higher grades of luxation and include recurrence of luxation, implant migration, patellar tendinopathy, tibial tuberosity avulsion, patellar ligament rupture and infection, with post-operative luxation being the most common complication [8–11]. Given the association between patellar tendon length and MPL, it is possible that elongation of the patellar tendon following surgery could also predispose to post-operative luxation. The lengthened patellar tendon could potentially result in the patella alta, and if concurrent skeletal and soft tissue abnormalities exist, affected animals might be at an increased risk of luxation. To the author’s knowledge, changes in patellar tendon length following surgical correction of MPL have not been quantified.

The purpose of this study is to radiographically quantify changes in patellar tendon length following common MPL corrective surgeries and evaluate potential risk factors that may contribute to post-operative morbidity. We hypothesize that the patellar tendon would lengthen after surgery, and would be associated with high grades of MPL (grade 3 or 4), older age, and body weight. We also hypothesized that patellar tendon lengthening might be associated with recurrence of luxation in dogs with relatively normal position of the patella.

Material and methods

Case selection and medical record review

Medical records were searched for dogs that had previously undergone surgical correction for medial patellar luxation between March 2008 and December 2017. All surgeries were performed by a board-certified veterinary surgeon. Dogs that had concurrent stifle disease, such as cranial cruciate ligament rupture, were excluded from this study. Dogs with lateral patella luxation were not included in the study. Dogs were included if they had appropriate radiographic follow up including pre-operative, immediately post-operative, and follow up radiographs of 2–3 months. Data collection included the animal’s age, weight, grade and side of MPL, procedure(s) performed and any post-operative recurrence of luxation. Surgical treatments included tibial tuberosity transposition, distal femoral osteotomy, trochleoplasty (trochlear wedge or block recession), medial release, and lateral imbrication.

Radiographic assessment

The evaluation of digital stifle radiographs in this study focused on the length of the patellar tendon. All measurements were performed on a standard mediolateral radiographic projection of the stifle using a dedicated PACS workstation using DICOM viewing software (Merge Healthcare Inc, Chicago, Illinois). All dogs were under general anesthesia for initial radiographs and all subsequent radiographs were performed under sedation. The radiographs were judged as satisfactory if superimposition of the femoral condyles was achieved, and if it included the metaphyseal-diaphyseal junction of both the femur and tibia. The patella length was measured as the distance from the most proximal aspect to the most distal aspect of the patella. The patellar tendon length was measured as the distance from the most distal aspect of the patella to the most proximal aspect of the tibial tuberosity (Fig 1). These measurements are consistent with those performed in previous studies that investigated MPL and associated changes in patellar tendon length in medium to giant breed dogs [4]. Care was taken to not include osteophytes when noted at the distal aspect of the patella. The patellar tendon length to patellar length ratio (PTL:PL) was then calculated. For each case, the initial digital measurements made pre-operatively were juxtaposed next to additional projections at other time points to ensure use of precisely the same landmarks for all measurements. This methodology aided in consistent measurements, particularly in cases where implants interfered with bony landmarks. Because the measurement was a ratio, calibration of the images was not necessary and thus was not performed. The angle between the femur and tibia was not measured, as previous studies have demonstrated no association between the degree of stifle flexion and two methods of calculating PTL:PL [12]. All measurements were made by one of the investigators (KD). The investigator was trained to take appropriate radiographic measurements prior to the start of the study.

Fig 1. Pre-operative (A) and 3 month post-operative (B) stifle radiographs in a dog with MPL correction.

Fig 1

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Mild elongation of the patellar tendon is evident when comparing the pre-operative patellar tendon length (blue line) to the post-operative patellar tendon length (black line). Patellar tendon length: patellar length ratio was calculated by dividing the patellar tendon length over the length of the patella (white line).

Statistical analyses

Descriptive statistics were reported as mean ± standard deviation. Unconditional odds ratios were calculated to evaluate for relationships among various potential risk factors. The specific odds comparisons performed included: trochleoplasty vs elongation, age (<1 year) vs elongation, age (≥1 year) vs reluxation, weight (<10 kg) vs elongation, weight (≥ 10 kg) vs reluxation, grade of MPL vs elongation, and reluxation vs elongation. For these models, elongation was defined as ≥ 5% increase in the 2–3 month postoperative PTL:PL ratio compared to preoperative values. The percentage of elongation was chosen based on previous studies that quantified PTL:PL ratios in control dogs compared to those with MPL. The study demonstrated a difference of approximately 5% in PTL:PL ratios between the upper limit of confidence interval of control dogs compared to the lower limit of the confidence interval for dogs affected by MPL [4]. In addition, this magnitude of change was well above the previously reported errors (approximately 2%) associated with the radiographic measurements. All statistics were calculated using GraphPad Prism version 8.0 (San Diego, California).

Results

Of a total of 242 stifles surgically repaired, only 50 cases met the inclusion criteria. Mean age at the time of surgery was 2.5 ± 1.8 years, ranging from 0.5–7.7 years. Of the 50 cases, 10 dogs were less than 1 year of age and the remainder were 1 year of age or older. Mean body weight was 13.9 ± 11.0 kg ranging from 1.0 kg– 34.4 kg. Of the 50 cases, 26 dogs were less than 10 kg and the remainder weighed 10 kg or more. The distribution of MPL grade was 32% grade II, 60% grade III, and 8% grade IV. The average pre-operative PTL:PL ratio was 2.06 ± 0.32, and the average 2–3 month post-operative PTL:PL ratio was 2.05 ± 0.29. Of the 50 cases, 46% of dogs had patellar tendon lengthening and 54% of dogs had shortening at the 2–3 month follow up point. Of the 50 cases, 10 of the dogs had patellar tendon lengthening of ≥ 5% and 11 of the dogs had patellar tendon shortening of ≥ 5% at the 2–3 month follow up point. Age, body weight, and grade of MPL were not significantly associated with risk of patellar tendon elongation (Table 1). Patellar tendon lengthening was not associated with recurrence of MPL (Table 1). The distribution of percent change in PTL:PL ratios from pre-operative to final follow up measurements are included in Fig 2.

Table 1. Odds ratio calculations for potential risk factors associated with patellar tendon elongation and recurrence of patellar luxation.

Parameter Odds Ratio 95% Confidence interval Z statistic P-value
Trochleoplasty vs elongation 0.3382 0.0636–1.7995 1.271 0.2037
Age (<1 year) vs elongation 0.5 0.1169–2.1392 0.935 0.35
Age (≥1 year) vs reluxation 0.4355 0.1005–1.8879 1.111 0.2666
Weight (<10 kg) vs elongation 0.3878 0.0876–1.7172 1.248 0.2121
Weight (≥10 kg) vs reluxation 0.2045 0.0385–1.0866 1.862 0.0625
Grade of MPL vs elongation 0.6429 0.1531–2.6986 0.604 0.5461
Reluxation vs elongation 2.4287 0.4869–12.1138 1.082 0.2792
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Fig 2. Distribution of percent change in PTL:PL ratios.

Fig 2

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The percent change was calculated using pre-operative PTL:PL measurements compared to final follow-up measurements.

Discussion

The patellar tendon is subjected to changes following surgical procedures involving the stifle. Retrospective studies in human have demonstrated an increased prevalence of patellar tendon shortening with total knee arthroplasties and increased prevalence of patellar tendon lengthening with unicompartmental knee arthroplasty at 5 years post-surgery [13]. Previous investigators have demonstrated reduced blood flow to the patellar tendon following TPLO and patellar luxation procedures in cadaveric canine models [14]. Previous studies have established that large breed dogs with MPL have a relatively longer patellar tendon compared to dogs without stifle disease [4]. Other investigators have demonstrated shortening of patellar tendon length following tibial plateau leveling osteotomy procedures in dogs [15]. Although patella alta has previously been proposed as predisposing factor to post-operative recurrence of luxation, other studies have demonstrated this condition in healthy dogs without evidence of orthopedic disease [12, 16]. These factors in addition to other underlying soft tissue and skeletal abnormalities may all contribute to changes in patellar tendon length.

The main findings of our study show that patellar tendon length can increase or decrease following MPL procedures. However, these observations were not significantly associated with the any of the risk factors we investigated, such as age, body weight and grade of MPL. Furthermore, there was no association between postoperative lengthening of the patellar tendon with luxation recurrence.

For the purposes of our investigation, we defined elongation as a 5% or greater increase in the PTL:PL ratio. The 95% CI of the PLL:PL ratio observed in previous studies was 1.92–2.03 for large breed dogs with MPL [4]. This is similar to our average pre-operative PLL:PL ratio of 2.06, and our average 2–3 month follow up ratio of 2.05. Since our study population included both large and small breed dogs, it may be more representative of the broader population of dogs affected by MPL when compared to previous investigations [4].

Dogs are considered skeletally mature by 1 year of age and at this point the structures of the quadriceps mechanism should be completely developed. Therefore, we investigated the association between <1 year of age and risk of elongation >5%, and the association between >1 year of age and risk of luxation recurrence. Although reluxation was noted in some cases, there was not a significant association between age > 1 year and risk of patellar reluxation. These findings are consistent with other retrospective studies that found no association between age and risk for developing complications following MPL procedures [8]. Although the PTL:PL ratio increased in 23/50 cases at 2–3 months post-operatively, patellar elongation did not appear to be influenced by age in our study.

Recurrence of patellar luxation and other post-operative complications following MPL corrective surgeries have been associated with in higher grade MPL, where there may be more tension on the quadriceps mechanism after surgery [8–10]. Of the 50 included cases, 9 had post-operative recurrence of luxation. Of these 9 cases with post-operative patellar luxation, three dogs had an MPL grade of 2, three dogs had an MPL grade of 3, and one dog had an MPL grade of 4. For the purposes of our investigation, we defined higher grades of MPL as grade 3 or 4. Although there was lengthening observed in higher grades of MPL, there was no significant association between grade of MPL and risk of elongation >5%. This may be due to the fact that the amount of lengthening that occurred was not substantial enough to have the patellar positioned proximal to the trochlear groove which contributes to the potential for recurrence of luxation. Another consideration could be pre-existing elongation of the patellar tendon that is subjected to abnormal tension during development and physical activity. Based on these findings, a long patellar tendon appears to be more clinically relevant if it is pre-existing, rather than as a consequence of surgery.

Other investigators have also detected a correlation between body weight and risk of recurrence of luxation following MPL corrective procedures [9, 11]. The average body weight of dogs included in our investigation was lower than those reported in other studies investigating PLL:PL [4, 12]. This is due to the fact that the aforementioned studies only included populations of large breed dogs, while our study population included both large and small breed dogs. Although there was observed lengthening as well as shortening, there was no significant association between weight small (< 10 kg) vs medium-large dogs (>10 kg) and risk of elongation. Additionally, there was no association between size and risk of recurrence of luxation.

The retrospective nature of this investigation was a limitation of this study. This could have contributed to some inaccuracies regarding the presence of lower grade luxation recurrence or other mild post-operative complications. The short period of follow-up of 2–3 months is another limitation. The lack of radiographic follow-up was the main reason many cases did not qualify for this study. Future prospective studies could include longer term follow up to assess the change in PLL:PL ratios over time in conjunction with more accurate clinical assessment. Another limitation of the study was the inability of blinding of the images when acquiring the measurements. However, images were compared within each specimen to ensure consistent landmarks were used. Measurements were only performed once per image by a single observer, and use of multiple measurements may have improved the accuracy of the study. This was not deemed necessary because a previous study demonstrated low intra- and inter-observer variability of PLL [4, 15]. Other limitations include the lack of control groups for comparison and the wide breed variability. In this study, dogs were categorized based on weight and specific breeds were not taken into account. Future studies could assess whether patellar elongation is more clinically relevant in specific breeds.

Lengthening and shortening of the patellar tendon can occur both small and large breed dogs following common MPL procedures. Our investigation revealed that these changes are not associated with age, body weight or grade of MPL, nor do they influence the risk of luxation recurrence in the short-term. Further investigation of these parameters is warranted to characterize the changes in patellar tendon length over a longer follow-up period.

Supporting information

S1 Table

(XLSX)

Click here for additional data file. (33.3KB, xlsx)

Data Availability

All relevant data are within the manuscript and its Supporting Information files.

Funding Statement

There has been no significant financial support for this work that could have influenced its outcome.

References

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Decision Letter 0

Silvia Sabattini

Transfer Alert

This paper was transferred from another journal. As a result, its full editorial history (including decision letters, peer reviews and author responses) may not be present.

11 Jun 2020

PONE-D-20-13783

Radiographic evaluation of patellar tendon length following corrective surgical procedures for medial patellar luxation in dogs.

PLOS ONE

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Reviewer #1: Dear Authors, I have been really pleased to review this manuscript. I think patellar luxation is one of the most important disease that any orthopaedic surgeon has to deal with. Although we have a huge kwnoledge of this condition, I belive there is still a lot of to understand to improve our diagnosis, treatment and outcome. For this reason any information that might help reaching this goal is very valuable.

Regarding the study presented here, looking at the changings of the patellar tendon and its effect on the vertical position of the patella, I think this is an interesting point of view that has still not been properly studied.

I don't have many critics to do about the aim of the paper, the statistical analysis and the contents in general. I have given some suggestions to try to improve the text, and I have tried to find potential conflicts that need to be addressed to make the manuscript sound. My biggest critic is regarding references since PL is a quite popular topic in veterinary literature and when I read a paper on this topic I expect to find a lot of scientific support.

I would also consider to find some evidence of the same condition in human literature, it might be interesting for comparing your results.

My general judgment is positive. Following you can find some specific comments. I hope my suggestions will help you in publishing this paper.

COMMENTS:

39 – correct “patellar” to “patella”

40-42 – Mostafa 2008 seems to be the only paper used as reference for this study. See comments later.

47-50 – more references.

50-52 – confusing, reporting some info as on line 40-42

52-54 – this is probably non-relevant information. The paper deals with MPL and any other affection of the stifle lead to exclusion. I would erase it.

54-56 – please discuss, for a better understanding of the Reader, why this can happen.

62-63 – please change “would” to “might”. Your hypothesis is correct from my point of view; however have you considered that some dogs, especially some small breed, can have concomitant MPL and patella baja, and in this case the elongated patellar tendon should improve the tracking and not increase the risk of luxation. Maybe you should specify that the hypothesis is limited to dog with a relatively normal position of the patella.

71 – is 2-3 months an appropriate follow up? Patellar tendon stretching and elongation might take longer, also some complications as reluxation might happen later in time.

71-73 – please remove “from medical records”; correct “include” to “included”; please remove “comorbidities” because you stated previously your inclusion/exclusion criteria; change “surgical procedure(s) and post-operative complications” (let’s follow an appropriate chronologic order)

73 – Please change “Surgical corrective procedures performed include” to “Surgical treatment included”

78 – the reference to the Figure should be moved at the end of the sentence that describes the methodology of measurement.

77-80 – I am not convinced of the first sentence, it does not sound well. I would suggest to change this part as follow “All measurements were performed on a standard mediolateral radiographic projection of the stifle by using a dedicated PACS workstation using DICOM viewing software (Merge Healthcare Inc, Chicago, Illinois).”

80-81 – Can you mention the angle between femur and tibia? Do you think that limb positioning (in this case the degree of flexion) may affect measurement and result? (for replicating your study it is important to give to the Reader all the relevant information).

Maybe this can be helpful: “There was no association between the degree of stifle joint flexion and PLLj/PL and PLLm/PL ratios, respectively” (Łojszczyk-Szczepaniak, A., Silmanowicz, P., Komsta, R. et al. Determination of reference values and frequency of occurrence of patella alta in German shepherd dogs: a retrospective study. Acta Vet Scand 59, 36 (2017). https://doi.org/10.1186/s13028-017-0304-1)

82 – remove “from”, written twice

82-86 – vertical position of the patella in dogs has been previously reported and several methods have been described (for example Mostafa et al. and Johnson et al. described two different measurements). Can you please add a reference to your methodology in order to help the Reader to understand that not all the published studies are comparable between each other?

86-88 – this is a good way to reduce the risk of error. My recommendation for next time for more reliable data is to make repeated measurement and check for intraobserver difference in order to be sure that final number is as much precise as possible.

88 – regarding landmarks, have you noticed any implant interference in identifying the correct landmark? The accuracy of measurement might be discussed.

110 – “On a total of 242 stifles surgically repaired, only 50 cases met the inclusion criteria”. Maybe sounds better.

111 – “The majority of cases were excluded due to inadequate radiographic follow up”. I think you can delete this sentence since you stated previously your inclusion/exclusion criteria.

134 – Here I would mention a sentence that I suggested to remove in the introduction regarding the reported changings following TPLO procedure. I would start the discussion stating that the patellar tendon is subjected to changings and that some pathologies as well treatments can affect its length. Try to find further references (also check human literature) to strengthen this statement because this is the pillar of the paper.

139-142 – Please check the literature, there are papers that provide some information that are in contrast with studies you cited. It is important in the discussion review all the available literature to find evidence. “Additionally, patella alta has previously been proposed as predisposing factor to post-operative recurrence of luxation (Johnson 2006)”, but patella alta has also been observed in healthy dogs that did not exhibit orthopaedic problems in the stifle joints (Łojszczyk-Szczepaniak, A., Silmanowicz, P., Komsta, R. et al. Determination of reference values and frequency of occurrence of patella alta in German shepherd dogs: a retrospective study. Acta Vet Scand 59, 36 (2017)).

If you do not clarify this statements it sounds like that if the patella is alta you can have reluxation. I believe this complication is more related to other abnormalities rather than a patella higher than normal.

Wandagee 2013 reported “The outcome of surgery was considered good for grade II luxation with a 100% success rate. Recurrent medial patellar luxation was diagnosed in approximately 11% of dogs with grade III and in 36% of dogs with grade IV luxation.”

By reading the available literature, we have evidence that risk of reluxation is relatively low and patella alta might not be as relevant as you stated. And reluxation is generally observed in dogs with severe stifle abnormalities (both skeletal and of the soft tissues)

“Previous studies have established that large breed dogs with MPL have a relatively long patellar tendon compared to dogs without stifle disease (Mostafa 2008)”. This statement is correct, but remember that MPL is typically seen in small breed dogs.

160 – Please add more references.

161-167 – This is probably one of the most interesting part of the paper. You must answer this question “Why grade 3-4 MPL have not significantly different ratios?”. This is probably due to the pre-existing elongation of the tendon that is subjected to abnormal tension during growth or normal physical activity.

177-190 – Very good paragraph that answer some of my previous comments. Last sentence in incomplete.

Reviewer #2: Dear authors this topic is very interesting, and the study is well done but some issues must be revised. please find attached the comments.

Comment 1) Please add the name of the second co-author (all co-authors) at the beginning, in the "Order of Authors".

Comment 2) According to the journal guidelines, the references should be cited as the numbers in brackets. They should be numbered in the order they appear in the text. Please revise your citations.

Comment 3) According to the journal guidelines, the abstracts should not include abbreviations if it is possible.

Comment 4) Line 39: Please revise the words "patellar alta" to "patella alta".

Comment 5) Line 68-69: You did not talk about LPL in your study. Please explain if you excluded the dogs with LPL from your study. If LPL affected dogs were not excluded please explain the dogs with MPL and LPL. Is there any relationship between the dogs with bidirectional patellar luxation and post-operative patellar tendon length?

Comment 6) Please talk about the muscle contracture in your study and if the radiographs were taken under general anesthesia.

Comment 7) Lines 84 – 85. Please revise the sentence "the distance from the distal most aspect of the patellar to the proximal aspect of the tibial tuberosity" as "the distance from the most distal aspect of the patella to the most proximal aspect of the tibial tuberosity".

Comment 8) Lines 93 - 94. Please revise the sentence "Patella tendon: patellar ratio" as "Patellar tendon length: patellar length ratio".

Comment 9) Please include the name and version of the used statistical software and calculation methods (name of the tests) in the material and methods.

Comment 10) Please discuss the included dog breeds in your study. You only mentioned in the discussion that you had small to large breed dogs; it would not be out of interest if you could explain the frequency of each breed.

Comment 11) Line 158. You are talking about post-operative complications reported in the literature. Did you have any post-operative complications in included dogs? Please discuss it.

Comment 12) Line 185. Please discuss the level of experience of the observer who measured the alignments. Did the observer train to measure alignments before the study?

Comment 13) Line 190. The sentence "another limitation of the study was" is incomplete, please delete it.

Best regards

**********

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Reviewer #1: No

Reviewer #2: No

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PLoS One. 2020 Sep 4;15(9):e0238598. doi: 10.1371/journal.pone.0238598.r002

Author response to Decision Letter 0

9 Jul 2020

Dear PLOS ONE Editors and Reviewers,

Thank you for the critiques and suggestions for our manuscript. The majority of the recommendations made have added clarification and quality to the discussion of our study. We feel that the revised manuscripts have adequately addressed the concerns raised during the review process. Please see review the following rebuttal letter that addresses suggestions made. We hope that the revised manuscript is suitable for publication and look forward to hearing from you in due course.

Best, Kevin de Moya

1. Please ensure manuscript meets PLOS ONE’s style requirements including those for file naming

• The manuscript formatting has been updated including references and requirements made for file naming.

2. Please include your amended funding statements within your cover letter.

• See updated cover letter.

3. Please amend the manuscript submission data (via Edit Submission) to include author Stanley Kim.

• See amended manuscript.

4. Please include captions for your Supporting Information files at the end of your manuscript, and update any in-text citations to match accordingly.

• See amended manuscript.

5. Review comments to Author (Reviewer #1)

• Comment- Correct “patellar” to “patella” (line 39).

• Response- As suggested, this correction has been made (line 39).

• Comment- Add more references (line 47-50).

• Response- As suggested, additional references have been added to support the statements made (line 46-50).

• Comment- Confusing, reporting some info as on line 40-42 (line 50-52).

• Response- This sentence has been deleted.

• Comment- This is probably non-relevant information. The paper deals with MPL and any other affection of the stifle lead to exclusion. I would erase it (line 52-54).

• Response- As suggested, this sentence has been deleted.

• Comment- Please discuss, for a better understanding of the Reader, why this can happen (line 54-56).

• Response- As suggested, an additional sentence has been added to further clarify the statements made (line 51-53).

• Comment- Please change “would” to “might”. Your hypothesis is correct from my point of view; however have you considered that some dogs, especially some small breed, can have concomitant MPL and patella baja, and in this case the elongated patellar tendon should improve the tracking and not increase the risk of luxation. Maybe you should specify that the hypothesis is limited to dog with a relatively normal position of the patella (line 62-63).

• Response- As suggested, an addition to the hypothesis was added clarifying that our hypotheses apply to dogs with relatively normal patella positions (57-60).

• Comment- Is 2-3 months an appropriate follow up? Patellar tendon stretching and elongation might take longer, also some complications as reluxation might happen later in time (line 71).

• Response- 2-3 months is the last radiographic follow up time point before animals are discharged to resume normal activity, assuming no complications occurred. Although we suspect that patellar tendon lengthening can occur past this time point, we do not enough cases with radiographic follow up past this time point.

• Comment- Please remove “from medical records”; correct “include” to “included”; please remove “comorbidities” because you stated previously your inclusion/exclusion criteria; change “surgical procedure(s) and post-operative complications” (line 71-73).

• Response- The statement has been updated to reflect suggestions made (line 69-71).

• Comment- Please change “Surgical corrective procedures performed include” to “Surgical treatment included” (line 73).

• Response- The statement has been updated to reflect suggestions made (line 70-71).

• Comment- The reference to the Figure should be moved at the end of the sentence that describes the methodology of measurement (line 78).

• Response- The reference to Figure 1 has been moved to the end of the sentence that describes the measurement methodology (line 83).

• Comment- I am not convinced of the first sentence, it does not sound well. I would suggest to change this part as follow “All measurements were performed on a standard mediolateral radiographic projection of the stifle by using a dedicated PACS workstation using DICOM viewing software (Merge Healthcare Inc, Chicago, Illinois)- (line 77-80).

• Response- The statement has been updated to reflect suggestions made (line 75-77).

• Comment- Can you mention the angle between femur and tibia? Do you think that limb positioning (in this case the degree of flexion) may affect measurement and result? (for replicating your study it is important to give to the Reader all the relevant information)- (line 80-81).

• Response- An additional statement has been added with proper reference justifying why the degree of stifle flexion was not included in the study (92-94).

• Comment- Vertical position of the patella in dogs has been previously reported and several methods have been described (for example Mostafa et al. and Johnson et al. described two different measurements). Can you please add a reference to your methodology in order to help the Reader to understand that not all the published studies are comparable between each other? (line 82-86).

• Response- The statement has been updated to reflect that the measurements made are consistent with those of previous studies (83-85).

• Comment- Regarding landmarks, have you noticed any implant interference in identifying the correct landmark? The accuracy of measurement might be discussed (line 88).

• Response- An additional statement has been added describing the methodology used in cases where there was implant interference with bony landmarks (89-91).

• Comment- “On a total of 242 stifles surgically repaired, only 50 cases met the inclusion criteria”. Maybe sounds better (line 110).

• Response- The statement has been updated to reflect suggestions made (line 117).

• Comment- “The majority of cases were excluded due to inadequate radiographic follow up”. I think you can delete this sentence since you stated previously your inclusion/exclusion criteria (line 111).

• Response- As suggested, this statement has been deleted.

• Comment- Here I would mention a sentence that I suggested to remove in the introduction regarding the reported changings following TPLO procedure. I would start the discussion stating that the patellar tendon is subjected to changings and that some pathologies as well treatments can affect its length. Try to find further references (also check human literature) to strengthen this statement because this is the pillar of the paper (line 134).

• Response: This paragraph has been updated to reflect both veterinary and human studies that investigate changes in patellar tendon length as an introduction to our discussion. The veterinary studies that were previous mentioned in the introduction were moved to this section (141-153).

• Comment- Please check the literature, there are papers that provide some information that are in contrast with studies you cited. It is important in the discussion review all the available literature to find evidence. “Additionally, patella alta has previously been proposed as predisposing factor to post-operative recurrence of luxation (Johnson 2006)”, but patella alta has also been observed in healthy dogs that did not exhibit orthopaedic problems in the stifle joints (Łojszczyk-Szczepaniak, A., Silmanowicz, P., Komsta, R. et al. Determination of reference values and frequency of occurrence of patella alta in German shepherd dogs: a retrospective study. Acta Vet Scand 59, 36 (2017)). If you do not clarify this statements it sounds like that if the patella is alta you can have reluxation. I believe this complication is more related to other abnormalities rather than a patella higher than normal (line 139-142).

• Response- As suggested, the statement has been updated with further clarification and additional references (line 149-153).

• Comment- Add more references (line 160).

• Response- As suggested, this paragraph has been updated with additional references (line 176).

• Comment- This is probably one of the most interesting part of the paper. You must answer this question “Why grade 3-4 MPL have not significantly different ratios?”. This is probably due to the pre-existing elongation of the tendon that is subjected to abnormal tension during growth or normal physical activity (Line 161-167).

• Response- As suggested, this paragraph has been updated to give a proposal of why grade 3-4 MPL may not have significantly different ratios (181-183).

6. Review comments to Author (Reviewer #2)

• Comment- Please add the name of the second co-author (all co-authors) at the beginning, in the “order of authors”.

• Response- The name of the second co-author has been added in the “order of authors” section.

• Comment- According to the journal guidelines, the references should be cited as the numbers in brackets. They should be numbered in the order they appear in the text. Please revise your citations.

• Response- The references have been updated and are cited as numbers within brackets and are in the order they appear in the text.

• Comment- According to the journal guidelines, the abstracts should not include abbreviations if it is possible.

• Response- The abstract has been updated and does not include any abbreviations.

• Comment- Please revise the word “Patellar alta” to “Patella alta” (line 39).

• Response- As suggested, the statement has been updated to reflect suggestions made (line 39).

• Comment- You did not talk about LPL in your study. Please explain if you excluded the dogs with LPL from your study. If LPL affected dogs were not excluded please explain the dogs with MPL and LPL. Is there any relationship between the dogs with bidirectional patellar luxation and post-operative patellar tendon length? (line 68-69).

• Response- A statement has been added to the Methods section that clarifies dogs with lateral patellar luxation were excluded from the study (line 66-67).

• Comment- Please talk about the muscle contracture in your study and if the radiographs were taken under general anesthesia.

• Response- As suggested, an additional statement has been added regarding anesthesia and sedation for radiographs (line 77-78).

• Comment- Please revise the sentence "the distance from the distal most aspect of the patellar to the proximal aspect of the tibial tuberosity" as "the distance from the most distal aspect of the patella to the most proximal aspect of the tibial tuberosity" (line 84-85).

• Response: The statements have been updated to reflect suggestions made (line 80-83).

• Comment- Please revise the sentence "Patella tendon: patellar ratio" as "Patellar tendon length: patellar length ratio" (line 93-94).

• Response- The statement has been updated to reflect suggestions made (line 86-87).

• Comment- Please include the name and version of the used statistical software and calculation methods (name of the tests) in the material and methods.

• Response- A statement has been added to reflect the suggestions made (line 114-115).

• Comment- Please discuss the included dog breeds in your study. You only mentioned in the discussion that you had small to large breed dogs; it would not be out of interest if you could explain the frequency of each breed.

• Response- Dogs were categorized based on weight (<10kg vs >10kg) and specific breeds were not recorded. The sentence has been updated to accurately reflect data collected on each animal (line 69-70).

• Comment- Please discuss the level of experience of the observer who measured the alignments. Did the observer train to measure alignments before the study? (line 185).

• Response- An updated statement in the methods section is included to describe the training involved in taking radiographic measurements prior to the start of the study (line 94-96).

• Comment- You are talking about post-operative complications reported in the literature. Did you have any post-operative complications in included dogs? Please discuss it (line 158).

• Response- Additional sentences have been added that describe the number of dogs with post-operative patellar luxation and the associated grade of MPL for each dog (line 176-179).

• Comment- The sentence "another limitation of the study was" is incomplete, please delete it (line 190).

• Response- As suggested, this statement has been deleted.

Attachment

Submitted filename: Response to Reviewers.docx

Click here for additional data file. (20.3KB, docx)

Decision Letter 1

Silvia Sabattini

12 Aug 2020

PONE-D-20-13783R1

Radiographic evaluation of patellar tendon length following corrective surgical procedures for medial patellar luxation in dogs.

PLOS ONE

Dear Dr. Kim,

Thank you for submitting your manuscript to PLOS ONE and for this revised version. The reviewers have raised a few minor criticisms that can be easily addressed prior to publication.

Please submit your revised manuscript by Sep 26 2020 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

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We look forward to receiving your revised manuscript.

Kind regards,

Silvia Sabattini

Academic Editor

PLOS ONE

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: N/A

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Thank you for this revised version of the paper. All the previous comments have been addressed and now the paper is sound.

Minor comments:

23 - at least 1 month follow up; line 69, 108 is 2-3 months follow up. Although you say in the abstract "at least one month" and it is possible that none of the patients had only 1 month fu, it sounds strange. You could change in the text "2-3 months fu" with "the last fu" since doesn't matter if it is 2-3 months or longer or a bit lesser because in the inclusion criteria you stated "at least one month fu".

147 – change long to longer

206-207 – please cite the study

Reviewer #2: Dear Authors,

Thank you very much for submitting the revised manuscript. I do see that the requested edits were made and submitted successfully in the Editorial Manager. However, there are still some outstanding issues that must be resolved. Please revise the following parts.

Line 83) Please revise the word ''patellar'' to the ''patella''.

Line 178) Please revise the word ''glad'' to the word ''grade''.

Kind regards

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

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Decision Letter 2

Silvia Sabattini

20 Aug 2020

Radiographic evaluation of patellar tendon length following corrective surgical procedures for medial patellar luxation in dogs.

PONE-D-20-13783R2

Dear Dr. Kim,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Silvia Sabattini

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Acceptance letter

Silvia Sabattini

26 Aug 2020

PONE-D-20-13783R2

Radiographic evaluation of patellar tendon length following corrective surgical procedures for medial patellar luxation in dogs.

Dear Dr. Kim:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

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Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Silvia Sabattini

Academic Editor

PLOS ONE

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