Abstract
Objective
Total knee arthroplasty (TKA) is currently the best option for management of advanced knee arthritis for patients who have exhausted conservative management. There have been significant implant design improvements and this is a continuing process to help the surgeon replicate patient anatomy and kinematics. Amongst the many variables in implantation to achieve a well-functioning TKA, getting optimal femoral component sizing is one. Every implant system has certain discreet implant sizes and the surgeon has to strive to obtain the best fit possible for the patient and attain a well aligned and stable TKA. The aim of this study was to assess the frequency of various femoral component sizes being implanted with a system which has 2.5 mm antero-posterior increment between sizes, and to assess the incidence of anterior femoral notching when using a posterior referencing system.
Materials and methods
A retrospective analysis of 739 TKAs implanted in 532 patients between January 2013 and January 2016 at a single center using a single posterior stabilized implant system was done. Patient demographics and femur component size used was obtained from hospital patient records. Immediate post-operative radiographs were analyzed to look for anterior femoral notching and presence of this was classified according to Tayside classification. A telephonic follow up at minimum 2 tears post-surgery was done to interview for occurrence of supracondylar femur fracture or revision for any other causes.
Results
There were 207 bilateral and 325 unilateral TKAs performed in 532 patients during the study period. There were 245 males and 287 females with an average age of 61.3 years (43–81 years, SD = 7.2). The most commonly used femoral component was 60 mm and an intermediate size prosthesis was used in 43.11% patients. The incidence of femoral notching ranged from 0 to 6.3%. No patient had sustained a supracondylar condylar fracture at minimum 2 years follow up.
Conclusion
The availability of a larger number of femoral components in a TKA system allows the surgeon the modularity to choose and obtain the best fit possible. Restoration of posterior condylar offset, preventing anterior notching, medio-lateral overhang and patellofemoral joint stuffing are greatly dependent on correct femoral component sizing. The findings from our study underscore the need to use an implant system with as many femoral size options as possible with lesser increments in between sizes to minimize anterior femoral notching when using a posterior referencing technique.
Keywords: Arthroplasty, Replacement, Knee, Femur, Component, Fitting, Prosthesis, Treatment outcome
1. Introduction
Total knee arthroplasty (TKA) has become one of most common operations performed in orthopaedics. It has proved to be an amazing modulator in reducing pain, improving function and quality of life of patients and significantly reducing difficulty in performance of knee burdening activities in patients debilitated with advanced knee arthritis.1,2 The evolution of total knee prostheses available today has stemmed from design improvements to restore proper knee kinematics and duplicate patient anatomy. Surgeons had previously been required to adjust for the limited size options offered by manufacturers.3 The success of TKA depends to a large extent on prosthesis selection, accurate sizing and proper placement of the components.4,5 The Antero-Posterior (AP) and Medio-Lateral (ML) dimensions of femoral component are critical in deciding the implant size. AP diameter is important in maintaining flexion-extension spacing and optimal tension in the quadriceps mechanism.5 The universal expansion of standard off-the-shelf prosthetic choices and better instrumentation have allowed for more options to size appropriately.3
Some commonly used implant designs have an increment in AP femur sizing by 4 or 5 mm which might put the surgeon in difficulty if the patient requires an intermediate size for appropriate fit (Table 1). This leaves the surgeon with the choice to either downsize or upsize the implant to salvage the situation. Both downsizing and upsizing are associated with peculiar problems and pitfalls.
Table 1.
Comparison of anteroposterior dimensions of some commonly used Total Knee Arthroplasty implants.
| Implant | Manufacturer | Femur Size with AP dimension (mm) *Biomet denotes size as AP dimension | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| NexGen® | Zimmer | A | B | C | D | E | F | G | H | ||
| 45.5 | 49.5 | 53.5 | 57.5 | 61.5 | 65.5 | 70.5 | 76 | ||||
| PFC Sigma® | DePuy Synthes | 1.5 | 2 | 2.5 | 3 | 4 | 5 | 6 | |||
| 53 | 56 | 59 | 61 | 65 | 69 | 74 | |||||
| Vanguard® | Biomet | 55 | 57.5 | 60 | 62.5 | 65 | 67.5 | 70 | 72.5 | 75 | 80 |
| AGC® | Biomet | 55 | 60 | 65 | 70 | 75 | |||||
| Genesis® II | Smith & Nephew | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | ||
| 47.5 | 50.5 | 54.5 | 58.5 | 62 | 65.5 | 69.5 | 75 | ||||
| Anthem® | Smith & Nephew | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | ||
| 47 | 51 | 54 | 58 | 61 | 65 | 70 | 75 | ||||
| Scorpio NRG® | Stryker | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 11 | 13 | |
| 51 | 54 | 56 | 58 | 61 | 63 | 65 | 70 | 75 | |||
Patient-specific instrumentation and implants will be the next phase of the solution to individual variation.3 While navigation and customized implants have found recent interest in the knee arthroplasty marketplace, in a broad sense and in their current forms, these technologies have yet to reach their full potential in improving outcomes and patient experience.6 The availability of femur component with smaller differences in AP dimensions between sizes now being available can be a solution to this dilemma. The aim of this study was to assess the frequency of various femoral component sizes being implanted with a system which has 2.5 mm antero-posterior increment between sizes, and to assess the incidence of anterior femoral notching when using a posterior referencing system.
2. Material and methods
This is a retrospective observational study. A total of 714 patients underwent Total Knee Replacements at a single center by a single surgeon using a single implant design system (Vanguard® Complete Knee System, Biomet Inc. Warsaw, IN) between January 2013 and January 2016 for any etiology. The measured resection technique was used in all patients and a posterior referencing device with stylus placed on anterolateral cortex and external rotation dialed to stay parallel to the transepicondylar axis was used to decide the femoral implant size. Out of these 714 patients, 532 were accessible and consented for a telephonic follow-up and were included in the study. Data was collected and analyzed pertaining to demographics, size of femoral components used over this period and incidence of femoral notching on immediate post-operative radiographs as per Tayside classification.7
3. Results
The demographic data is summarized in Table 2. The total number of bilateral knee replacement cases in the study population was 207, with 85 males and 122 females and unilateral cases was 325, with 160 males and 165 females. The average age of the patients was 61.3 years (43–81 years, SD = 7.2). An intermediate size of femur implant (was in used in 319 knees, which accounts for 43.11% of the total knees evaluated (Fig. 1). The distribution of anterior femoral notching on lateral radiographs is described in Table 3.
Table 2.
Demographic data of study population.
| Total TKAs during the study interval | 714 |
|---|---|
| TKAs included in retrospective analysis | 532 |
| Bilateral TKAs | 207 |
| Male: Female 85: 122 | |
| Unilateral TKAs | 325 |
| Male: Female 160: 165 | |
| Age (in years) | 61.3 ± 7.2 (Range 53–81 years) |
Fig. 1.
The distribution of various femoral component sizes in the cohort, with total and sex-wise distribution.
Table 3.
Distribution of anterior femoral notching based on Tayside7 classification.
| Femur size | Total no. | Male | Female | Notching |
|||
|---|---|---|---|---|---|---|---|
| Grade I | Grade II | Grade III | Total percentage | ||||
| 55 | 156 | 32 | 124 | 3 | 3 | 0 | 3.8% |
| 57.5 | 166 | 57 | 109 | 6 | 2 | 0 | 4.8% |
| 60 | 197 | 76 | 121 | 8 | 4 | 0 | 6% |
| 62.5 | 136 | 48 | 88 | 5 | 2 | 1 | 5.8 |
| 65 | 61 | 38 | 23 | 2 | 0 | 0 | 3.2% |
| 67.5 | 16 | 10 | 6 | 1 | 0 | 0 | 6.3% |
| 70 | 5 | 4 | 1 | 0 | 0 | 0 | – |
| 75 | 2 | 2 | 0 | 0 | 0 | 0 | – |
At minimum 2 years follow up, none of the patients had sustained a supracondylar femur fracture of the femur. One patient had sustained a femoral neck fracture and another a bimalleolar ankle fracture, both of which were managed surgically. Two patients underwent debridement, poly exchange and retention of prosthesis for early infection. There was no surgery for component revision in any patient for any indication.
4. Discussion
Implantation of a TKR is currently performed by the either the conventional method or using patient specific instrumentation or aided by computer navigation. Whichever method is chosen, proper sizing and component placement in TKR is of significance to achieve optimum post-surgical outcomes and longevity of the implants. Sizing of the femoral component is dependent of a number of factors like the referencing technique used (anterior or posterior), implant system being used, axial rotation of the femoral component and site of stylus placement for sizing. The complications of wrong sizing are well known. Obtaining the exact size as the implant available depends on the implant system being used and number of sizes available. It is not unusual that the surgeon gets a reading which could be in between two sizes. In such a scenario, the dilemma is whether to oversize or undersize the component. When an anterior referencing system is used, oversizing can lead to a smaller flexion gap and the reverse happens when it is undersized, causing flexion instability. This can compromise balancing, stability and range of motion.
A posterior referencing system scores over an anterior system by creating consistent flexion spaces and restoring the posterior condylar offset and posterior condylar offset ratio.8, 9, 10, 11 In this case, undersizing can lead to anterior femoral notching, whereas oversizing can cause discomfort and limit range of motion due to overstuffing of patellofemoral joint.12,13 In spite of general agreement regarding the same, Beldman et al.8 showed that overstuffing the knee does not affect clinical outcome or anterior knee pain incidence. A review of the merits and demerits of both anterior and posterior referencing systems has been done by Charette et al. recently.14
The incidence of anterior femoral notching has been reported to be as high as 30%–40% when using a posterior referencing system.7,15 This rate is much lower in our study population, being a maximum of 6.3% in one size group. Anterior notching is supposed to create a stress riser, predisposing to the occurrence of supracondylar femur fracture. Hitt et al.16 stated that under-sizing may lead to exposing of bare areas of bone leading to osteolysis and implant failure. Culp et at reported than the torsional strength of distal femur reduced by 29% when the anterior femur cortex was violated by 3 mm.17 Lesh reported an 18% reduction of bending strength and 39% reduction of torsional strength due to notching and these femora were prone to a short oblique supracondylar fractures.18 The clinical significance of anterior femoral notching is still not proven. No patient in our study had this complication. Of the 200 patients in Gujarathi's study, only 1 patient with a Grade 2 notching sustained a traumatic supracondylar fracture.7 Ritter et al. also believed that notching did not increase the risk of supracondylar fractures.19
The anteroposterior sizing is also dependent on the rotation of the femoral component. All patients in this study had the component rotation dialed to stay parallel to the transepicondylar axis as is the protocol at our center. The transepicondylar axis has been shown to be a more reliable reference than posterior condylar axis for determining rotation in both varus and valgus knees.20 Koninckx demonstrated that as the external rotation is increased from 0° to 3° and 5° respectively, the AP sizing increased by 2.3 mm and 3.8 mm respectively using the same implant system as in our study.21 They did report the occurrence of medio-lateral overhang when a larger component was used due to the increased external rotation to match patient anatomy. Medio-lateral overhang is a potential problem, especially in females and in the Asian knees when implanting standard prostheses, thus necessitating narrow components in some cases.22, 23, 24 The site of placement of stylus on anterior femur also determines the size and this was the anterolateral cortex in all patients in this study. A point located centrally on the anterior cortex and 2 cm proximal to the margin of anterior condyle probably is the best point for accurate sizing.25
In recent days, the focus is growing towards patient specific knee systems and instrumentation. They are believed to decrease the incidence of improper sizing during TKR. The main concern in use of these systems is the cost of the knees and financial load over the healthcare system to sustain regular use of these knee systems. In developing countries, the cost factor is more significant as many of the patients who undergo TKR are not insured and pay for the procedure themselves. This is the main reason that patient specific knee systems have not yet become an acceptable option in practice.
In our study, we found out the incidence of Intermediate sizes used in TKR over 3 years to being 43.11% which is high by any standards. This result was comparable to the study by Lombardi et al.26 where they evaluated 1903 knees, 405 of which were done before the availability of intermediate size femoral components of the same implant system and procured data searching for incidence of intermediate sizes. After the availability of intermediate sizes, 31.6% knees had 62.5 mm and 7.5% knees had 67.5 mm femoral components implanted. They also concluded that postoperative functional scores were better in the knees after use of intermediate sizes as compared to before though they were not statistically significant. The introduction of implant systems with more sizing options and lesser difference in the AP dimensions of femoral components will give the surgeon some modularity to avoid improper sizing and thus avoiding its complications. Well-designed prospective randomized studies evaluating the functional and radiological outcome in knees where these intermediate sizes are used to ascertain this point. The aim of this study was to evaluate the incidence of use of intermediate sizes of femoral components of a TKR system and incidence of anterior femoral notching using a posterior referencing.
This study has several limitations which must be underscored. This is a retrospective observational study where one implant design which was implanted using a single technique, was analyzed to understand limited parameters. Any bias inherent to such a study design is applicable in our study. Since pre-operative functional scores were not available for the patients in hospital records, no such outcome measures were evaluated even at follow up. Hence, we are unable to state if availability of greater femoral size options would influence patient reported functional outcome. Only a telephonic interview was done at follow up and no clinical examination was done for this cohort. A radiographic comparison of these patients is also worthwhile and is subject of an ongoing activity at our center.
5. Conclusion
The availability of a larger number of femoral components in a TKA system allows the surgeon the modularity to choose and obtain the best fit possible. Restoration of posterior condylar offset, preventing anterior notching, medio-lateral overhang and patellofemoral joint stuffing are greatly dependent on correct femoral component sizing. The findings from our study underscore the need to use an implant system with as many femoral size options as possible with lesser increments in between sizes to minimize anterior femoral notching when using a posterior referencing technique.
Conflicts of interest
One author reports personal fees from ARTHREX, grants, personal fees and non-financial support from ZIMMER BIOMET, personal fees from SMITH & NEPHEW, personal fees from CONMED, other from JAYPEE MEDICAL PUBLISHERS, NEW DELHI, outside the submitted work.
Source(s) of support
Nil.
Presentation at a meeting
Not Applicable.
Conflicting interest (if present, give more details)
Dr. Sachin Tapasvi reports personal fees from ARTHREX, grants, personal fees and non-financial support from ZIMMER BIOMET, personal fees from SMITH & NEPHEW, personal fees from CONMED, other from JAYPEE MEDICAL PUBLISHERS, NEW DELHI, outside the submitted work. Other authors have nothing to declare.
Contribution details
Anshu Shekhar: Concepts, Design, Definition of intellectual content, Literature search, Clinical studies, Data acquisition, Data analysis, Manuscript preparation, Manuscript review, Guarant. Ch Chandra Krishna: Literature search, Clinical studies, Data acquisition, Manuscript preparation. Shantanu Patil: Concepts, Design, Definition of intellectual content, Literature search, Clinical studies, Data analysis, Statistical analysis, Manuscript preparation, Manuscript editing, Manuscript review, Guarant. Sachin Tapasvi: Concepts, Design, Definition of intellectual content, Clinical studies, Data acquisition, Manuscript preparation, Manuscript editing, Manuscript review, Guarant.
Acknowledgement (if any)
The authors would like to acknowledge the efforts of Dr. Sagar Kelkar, Research Fellow, for data collection related to this study.
Footnotes
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcot.2019.03.006.
Contributor Information
Anshu Shekhar, Email: dr.anshushekhar@gmail.com.
Ch Chandra Krishna, Email: chchandra.krishna@gmail.com.
Shantanu Patil, Email: shantanusp@gmail.com.
Sachin Tapasvi, Email: stapasvi@gmail.com.
Appendix A. Supplementary data
The following is the Supplementary data to this article:
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