Abstract
Background
Role of MAKOplasty software in determining femoral neck version, distal-femoral resection angle, tibial axis difference, distal-femoral rotation, medial/lateral tibial slope, and tibial tubercle alignment has yet to be fully explored.
Methods
Preoperative CT scans and plain films of 99 patients were obtained for each patient according to predetermined MAKO-protocol by four observers. Reliability analyses (Cronbach's Alpha-test) was performed to determine agreement between raters for angle measures.
Results
Anatomic measurements were similar to previously published literature, and cronbachs'alpha analysis demonstrated agreement amidst all observers.
Conclusion
MAKOplasty software produces similar results to anatomic measurements in planning for TKA with good reproducibility.
Keywords: Total knee arthroplasty, MAKOplasty, CT Scans, Observational case studies, Anatomic measurements, Reliability analysis
1. Introduction
Total knee arthroplasty (TKA) has become one of the most commonly performed operations in the United States with demand expected to grow to 3.48 million procedures by 2030.1 Long term survival and optimal function of TKA depend on achieving neutral postoperative mechanical alignment within 3° of the mechanical axis of the limb.2,3 The use of computed tomography (CT) scan is routinely used in the diagnosis and treatment of periarticular fractures and patellofemoral pathology. In adult reconstruction surgery, adoption of this technology has gained momentum, especially with the advent of robotic assisted knee replacement.10 The CT scan with 3D reconstruction is now a standard imaging modality by which surgeons preoperatively plan and evaluate lower limb alignment.4
In the preoperative phase, orthopaedic surgeons utilize CT scans to aid in appropriate positioning of both tibial and femoral implants.5 In the postoperative stage, the CT scan is routinely used as a tool in the evaluation of failed TKA especially rotational malalignment of the femoral components.6, 7, 8 Literature suggest that CT image-guided knee replacement provides near perfect alignment of the femoral and tibial component. One of the more popular robotic guided knee systems on the market, MAKO, utilizes patient specific preoperative CT images and provide 3-dimensional software. MAKO software allows surgeons to identify the bone resection depth, pre- and postoperative alignment, optimal component size, rotational considerations, as well as deformity correction prior to the exposure of the joint.9 The imaging is then calibrated to the patient's anatomy during the procedure by using anatomical landmarks.9
We present a simple observational case series assessing various anatomic angles measured in the preoperative phase of a total knee arthroplasty. We used both plain film and CT scan MAKO data to measure the following angles: femoral neck version, distal femoral resection angle, mechanical axis of the tibia, distal femoral rotation, medial and lateral tibial slope, and tibial tubercle alignment between various observers. We then compare our normative values to those cited within orthopaedic literature. Our hypothesis is that our values will be similar amidst observers, as well as those cited within orthopaedic literature.
2. Materials and methods
Our study was conducted at Level 1 urban academic trauma center. A total of 99 patients from 2016 to 2018 met inclusion criteria for the purposes of our study. All patients were diagnosed with uni, bi, or tricompartmental osteoarthritis of the knee. All patients were above the age of 18, skeletally mature, and demonstrated the ability to bear full weight on both lower extremities prior to surgery. Patients with a past history of revision total or unicompartmental knee arthorplasty, traumatic fracture to the ipsilateral lower extremity, neuromuscular disorder, and or presence of grossly osteoporotic bone or bone defects seen on preoperative radiographs were excluded from the study as these factors may significantly alter the natural alignment of the knee.
Preoperative long leg and computed tomography (CT) scans of the lower limbs were obtained for each patient according to our institution's MAKO protocol. Each scan was reviewed by the radiology technician to minimize measurement variability due to limb rotation. The protocol called for patients to be supine, feet first, with ipsilateral foot secured in an upright position. A kilovoltage peek of 120 kV and a 512 × 512 image resolution matrix were used. CT scans were 0.5–1 mm thick slices at 0.5–1 mm spacing for detailed analysis. Hip, knee, and ankle regions were scanned in the axial plane beginning at the hip through the knee and ending at and including the ankle joint.
Each CT scan was reviewed by four observers: three orthopaedic residents (2 nd year, 3rd year, and 4th year, respectively), as well as a fellowship trained adult reconstructive specialist. Each reviewer was blinded to the results obtained by any of the other three observers. Both plain film and CT scan MAKO data to measure the following angles: femoral neck version, distal femoral resection angle, mechanical axis of the tibia, distal femoral rotation, medial and lateral tibial slope, and tibial tubercle alignment between various observers.
3. Statistical analysis
The primary outcome of interest in this study was average scores of measured angles by different observers and the difference of measures among observers. There has yet to be a study that assesses native anatomy of patients undergoing TKA using MAKO software. Reliability analysis (Cronbach's Alpha test) was performed to determine the agreement between raters for angle measures. Fleiss Kappa analysis was performed to determine the agreement between raters for alignment measures. For all analyses with P value smaller than 0.05 was considered statistically significant. All analyses were performed using SPSS software (Version 25, IBM, Chicago, IL).
4. Results
A total of 36 males and 63 females met inclusion criteria for our study. The average age of males was 58.14 (9.03) years whereas females were 61.10 (SD 9.03) years old (Table 1). The average BMI within our patient population was 32.35, and no significant difference was noted between the male and female population (31.24 vs. 32; P = >.05) (Table 1).
Table 1.
Demographic information and baseline data.
| Criteria | n | Age | Standard Deviation | BMI | Standard Deviation | |
|---|---|---|---|---|---|---|
| Global | 99 | 60.02 | 9.45 | 32.35354 | 6.670530884 | |
| Gender | Male | 36 | 58.14 | 9.03 | 31.24722 | 4.971576352 |
| Female | 63 | 61.10 | 9.59 | 32.98571 | 7.166968521 | |
| Ethnicity | Black | 37 | 57.41 | 7.67 | 31.65135 | 6.496521361 |
| White | 26 | 61.81 | 9.95 | 30.88077 | 6.248329007 | |
| Hispanic | 1 | 75 | 22 | |||
| other | 14 | 58.29 | 10.31 | 37.02857 | 6.105411039 | |
| American Indian/Alaska Native | 2 | 66.5 | 2.12 | 29.75 | 6.576093065 | |
| Declined | 19 | 62.47 | 10.47 | 33.11053 | 6.770433659 | |
| Surgical Site | Left | 39 | 62.21 | 9.31 | 31.42821 | 5.532295075 |
| Right | 60 | 58.6 | 9.35 | 32.955 | 7.298094132 |
The femoral neck version was recorded by measuring the angle between the femoral neck axis (line from the femoral neck center to the femoral head center) and the trans-epicondylar femoral axis (a line running through lateral epicondylar prominence and the sulcus of the medial epicondyle). Table 2 demonstrates a mean femoral neck version of 3.61° anteversion of all subjects included within study. No significant difference was associated between male and female subjects (3.24 vs. 3.82, respectively; p = .532). Assessing laterality demonstrated a significant difference between right and left knees (4.31 vs. 2.53; p = .045). No significant differences were recorded between Caucasians and African American patients (4.22 vs. 3.16; p = .366).
Table 2.
Gender, laterality, and ethnic differences between femoral neck version angle.
| Femoral Neck Version Angle (°) | ||||
| Mean | Standard Deviation | P value | ||
| Global | 3.61 | 9.209 | ||
| Gender | Male | 3.24 | 8.585 | 0.532 |
| Female | 3.82 | 9.558 | ||
| Side | Right knees | 4.31 | 10.205 | 0.045 |
| Left knees | 2.53 | 7.323 | ||
| Ethnicity | African American | 4.22 | 9.79 | 0.366 |
| White | 3.15 | 8.686 | ||
The distal femoral valgus resection angle was defined as the angle between the anatomical axis of the femur (distal femoral center through femoral canal) and the mechanical axis of the femur (line from the femoral head center to the intercondylar notch of the distal femur). Table 3 displays the average valgus resection angle between our subjects as 5.51°. A significant difference was associated between male and female subjects (5.65 vs. 5.40; p = . 025). No significant differences were noted for laterality and ethnicity amidst subject (p = .62 and .41, respectively).
Table 3.
Gender, laterality, and ethnic differences between distal femoral valgus resection angle.
| Distal Femoral Valgus Resection Angle (°) | ||||
|---|---|---|---|---|
| Mean | Standard Deviation | P value | ||
| Global | 5.51 | 0.992 | ||
| Gender | Male | 5.65 | 0.999 | 0.025 |
| Female | 5.42 | 0.981 | ||
| Side | Right knees | 5.53 | 0.955 | 0.62 |
| Left knees | 5.47 | 1.05 | ||
| Ethnicity | African American | 5.3 | 1.078 | 0.408 |
| White | 5.4 | 0.95 | ||
The tansepicondylar axis was measured as the line that runs through the lateral epicondylar prominence and the sulcus of the medial epicondyle. Femoral rotation was measured as the angle between transepicondylar femoral axis and a flat horizontal axis. A positive femoral rotation angle indicated external femoral rotation. An average of 2.349° of external rotation was recorded for all subjects (Table 4). A significant difference was noted between males and females (2.015 vs. 2.538; p = .007) (Table 4). No significant differences were noted with regards to laterality and ethnicity (p = .609, p = .57, respectively) (Table 4).
Table 4.
Gender, laterality, and ethnic differences between femoral rotation.
| Femoral Rotation Angle (°) | ||||
| Mean | Standard Deviation | P value | ||
| Global | 2.349 | 1.8814 | ||
| Gender | Male | 2.015 | 1.8181 | 0.007 |
| Female | 2.538 | 1.894 | ||
| Side | Right knees | 2.31 | 1.8239 | 0.609 |
| Left knees | 2.409 | 1.9707 | ||
| Ethnicity | African American | 2.535 | 1.8899 | 0.57 |
| White | 2.658 | 1.5119 | ||
The lateral and medial tibial slopes were measured separately using angle between the center of their respective tibial plateau lines with a fixed rotational point at proximal tibial center (where center of canal in coronal and sagittal views meets most proximal tibia). The mean medial sided posterior slope was measured as 10.013 for all subjects. No significant difference was noted with regards to gender, laterality, and ethnicity (p = .738, .387, and 0, respectively) (Table 5). Table 6 demonstrates an average lateral posterior tibial slope of 10.205° for our patient sample. Differences between laterality were noted between right and left knees (9.84 vs. 10.766, respectively; p = .023). Ethnic differences were also significant with regards to lateral posterior slope between African Americans and Caucasians (10.887 vs. 9.717, respectively; p = .023) (Table 6).
Table 5.
Gender, laterality, and ethnic differences between medial posterior slope.
| Medial Posterior Tibial slope (°) | ||||
|---|---|---|---|---|
| Mean | Standard Deviation | P value | ||
| Total population | 10.013 | 6.3455 | ||
| Gender | Male | 9.888 | 4.3685 | 0.738 |
| Female | 10.084 | 7.2443 | ||
| Side | Right knees | 9.813 | 7.4177 | 0.387 |
| Left knees | 10.32 | 4.197 | ||
| Ethnicity | African American | 10.767 | 4.493 | 0 |
| White | 7.884 | 3.6522 | ||
Table 6.
Gender, laterality, and ethnic differences between lateral posterior tibial slope.
| Lateral Posterior Tibial Slope (°) | ||||
|---|---|---|---|---|
| Mean | Standard Deviation | P value | ||
| Total population | 10.205 | 3.9592 | ||
| Gender | Male | 10.599 | 4.1837 | 0.135 |
| Female | 9.979 | 3.8153 | ||
| Side | Right knees | 9.84 | 3.8368 | 0.023 |
| Left knees | 10.766 | 4.0895 | ||
| Ethnicity | African American | 10.887 | 4.475 | 0.023 |
| White | 9.717 | 3.6331 | ||
The mechanical axis of the tibia was defined as the line from the center of the knee to the center of the tibiotalar joint, while the anatomical axis of the tibia was a line from the ankle center through tibial canal. This study evaluated the difference between the two axes. The average difference between mechanical axis was defined as 0.71°. No differences were found between gender, laterality, and ethnicity amidst our patient population (p = .139, .126, and 0.169, respectively) (Table 7).
Table 7.
Gender, laterality, and ethnic considerations between varying tibial axes.
| Tibial Axis Difference (Anatomic – Mechanical) (°) | ||||
|---|---|---|---|---|
| Mean | Standard Deviation | P value | ||
| Global | 0.71 | 0.719 | ||
| Gender | Male | 0.78 | 0.848 | 0.139 |
| Female | 0.67 | 0.631 | ||
| Side | Right knees | 1.09 | 0.697 | 0.126 |
| Left knees | 0.92 | 0.662 | ||
| Ethnicity | African American | 0.72 | 0.688 | 0.169 |
| White | 0.61 | 0.645 | ||
Tubercle alignment was measured as a surrogate to tibial baseplate positioning. We defined tubercle alignment as the axial rotation of the tibial component using a line of best fit between the PCL attachment and the most medial aspect of the tibial tubercle. The tubercle was divided into fourths to adequately assess the current reference points in tibial rotation. Please see Fig. 1 for detailed information. Table 8 demonstrates an average alignment of 1.0. No significant differences were noted between tubercle alignment between male and female patients (p = .103). Right sided knees demonstrated a more externally rotated alignment of 1.09 compared to left sided knees which recorded 0.92 (p = .012). Though insignificant, African Americans also demonstrated more externally rotated tubercle alignment (1.08 vs. 0.92,; p = .091).
Fig. 1.
Tibial tubercle alignment utilizing Akagi's method and MAKO software.
Table 8.
Gender, laterality, and ethnic differences between tubercle alignment.
| Tubercle Alignment | ||||
|---|---|---|---|---|
| Mean | Standard Deviation | P value | ||
| Global | ||||
| Gender | Male | 1.1 | 0.769 | 0.103 |
| Female | 0.98 | 0.634 | ||
| Side | Right knees | 1.09 | 0.697 | 0.012 |
| Left knees | 0.92 | 0.662 | ||
| Ethnicity | African American | 1.08 | 0.714 | 0.091 |
| White | 0.92 | 0.746 | ||
Table 9 demonstrates statistical analysis amidst 4 observers who participated in our study. Cronbachs'alpha reliability analysis demonstrated that there was agreement among 4 raters for the angle measures of femoral neck version (alpha = 0.967), femoral rotation (alpha = 0.938), tubercale alignment (alpha = .789), medial posterior slope (alpha = .729), lateral posterior tibial slope (alpha = .966), femoral maxis (alpha = .804). Tibial mechanical axis demonstrated moderate reliability (alpha = .516).
Table 9.
Inter-Rater Reliability of Lower Extremity Biomechanical Angles using MAKO Software.
| Reliability | |||||
|---|---|---|---|---|---|
| Observer 1 (Z) | Observer 2 (M) | Observer 3 (P) | Observer 4 (PD) | Cronbach's Alpha | |
| Femoral Neck Version | 3.596 | 3.3131 | 4.1313 | 3.3939 | 0.967 |
| Femoral Rotation | 2.3673 | 2.3296 | 2.4592 | 2.3041 | 0.938 |
| Tubercle Alignment | 1.1111 | 0.9798 | 0.8384 | 1.1616 | 0.789 |
| Medial Posterior Tibial Slope | 11.0387 | 9.5892 | 9.2473 | 9.5387 | 0.729 |
| Lateral Posterior Tibial Slope | 10.1444 | 10.8182 | 10.8182 | 9.5677 | 0.966 |
| Femoral Resection Angle | 5.5859 | 5.6869 | 5.3636 | 5.3838 | 0.804 |
| Tibial Axis Difference | 0.5758 | 0.6263 | 0.5556 | 1.0707 | 0.516 |
5. Discussion
In this study a variety of anatomic measurements commonly used in planning a TKA were calculated using the MAKOplasty software. In efforts to evaluate the utility of MAKOplasty software, we compared our measurements to those existing in literature (Table 10, Table 11, Table 12). In our study we found that using MAKOplasty software produced similar results to what has been previously published in literature with regards to the distal femoral valgus resection angle, femoral rotation, and lateral and medial tibial slope. We calculated a mean femoral rotation angle of 2.3 ± 1.9° of external rotation. Femoral rotation values are known to vary depending on a surgeon referencing the transepicondylar axis or the posterior condylar axis (10, 11, 12, 13, 14). In addition, imaging modality and the inclusion or exclusion of cartilage remnants and/or osteophytes in the calculation (10, 11, 12, 13, 14). When comparing males and females we found females to have a statistically significant increased femoral rotation angle compared to males. It has been reported that female knees tend to be smaller in anterior-posterior and medial-lateral dimensions, may have thinner articular cartilage, and on average have more severe osteoarthritic wear than males.15, 16, 17, 18 These aforementioned factors may lead to an increased femoral rotation angle in females seen in our study. Despite there being some variability in this measurement in the literature, placing the femoral component within 3° of external rotation with respect to the posterior condylar line is common practice.
Table 10.
Tibial plateau literature review.
| Published Studies Regarding Tibial Slope | |||
|---|---|---|---|
| Author, Year of Publication | Present study | Ho, 2017 | Nunley, 2014 |
| Sample Size | 99 | 100 | Medial:2031 Lateral: 364 |
| Avergae Age | 60 | 54 | Not available |
| Pathology of Knee | Osteoarthritic | No Disease | Unicompartmental Osteoarthritic |
| Medial Tibial Slope | 10.0° ± 6.3 | 11.2° ± 3.2 | 6.8° ± 3.3° |
| Lateral Tibial Slope | 10.2° ± 3.96 | 10.9° ± 3.7 | 8.0° ± 3.3 |
| Population | American | Asian/Pacific Islander | Not available |
Table 11.
Femoral rotation literature review.
| Published Studies Regarding Femoral Rotation | ||||
|---|---|---|---|---|
| Author, Year of Publication | Present study | Meric, 2015 | Boisgard, 2002 | Thienpont, 2013 |
| Sample Size | 99 | 13,546 | 103 | 2637 |
| Avergae Age | 60 | 65.4 | 72.5 | 68 |
| Disease status of knees | Osteoarthritic | Osteoarthritic | Osteoarthritic | primary or post-traumatic osteoarthritis and rheumatoid arthritis |
| Distal Femoral Rotation Angle | 2.3° ± 1.9 | 3.3 ± 1.5° | 2.65° ± 1.9 | 4° ± 1.4 |
| Population | American | Not available | Not available | Four continent geographic distribution (without Asian populations) |
Table 12.
Distal femoral valgus resection angle review.
| Published Studies Regarding Distal Femoral Valgus Resection Angle | |||
|---|---|---|---|
| Author, Year of Publication | Present study | Meric, 2015 | Lee, 2015 |
| Sample Size | 99 | 13,546 | 952 |
| Avergae age | 60 | 65.4 | 72 |
| Modality | CT | CT | Long Leg Radiograph |
| Disease status of knees | Osteoarthritic | Osteoarthritic | Osteoarthritic |
| Distal Femoral Valgus Resection Angle | 5.5° ± 0.99 | 5.7° ± 2.3 | 7° ± 2 |
| Population | American | Not available | Asian |
Our findings suggest a distal femoral valgus resection angle of 5.51 ± 0.992° which is similar to what is reported in arthroplasty literature.19,20 We further compared the difference in distal femoral valgus resection angle between males and females and found that males had a slightly larger angle. This was also seen in a study by Deakin and colleagues who measured the distal femoral valgus resection angle in males and females using long leg standing radiographs (19). In practice it is accepted to make the distal femoral cut perpendicular to the mechanical axis of femur which on average is in 5–6° of valgus. However, there is evidence of anatomic variability in the distal femoral valgus resection angle which illustrates the importance of preoperative planning ensure the proper cut is made intraoperatively.19,20
A variety of measurements for tibial slope have been reported in the literature.21, 22, 23, 24 Insall and Kelly reported that the global posterior tibial slope was 10° relative to the tibial shaft.25 It was later shown that slopes of the medial and lateral tibial plateaus are not symmetrical and that these measurements have varied depending on the method of measurement used, presence of osteoarthritis, race, and sex.22, 23, 24 Using the MAKOplasty software we found a mean medial tibial slope to be 10.0 ± 6.3° and lateral tibial slope to be 10.2 ± 3.96°. With further analysis we also found our African American patients to have greater medial and lateral tibial slopes than our Caucasian patients, 10.767 ± 4.493 vs. 7.884 ± 3.652 and 10.887 ± 4.475 vs 9.917 ± 3.633, respectively. Similar findings have been reported in both African-American and Caucasian populations (24). At this time there is no widely accepted value for tibial slope that is used intraoperatively but values ranging from 3 to 7° are commonly used. This suggests that further research may be warranted to improve our understanding of tibial slope in relation to total knee arthroplasty outcomes.
It has been previously described that the mechanical and anatomic axis of the tibia should coincide in normal knees .26 Our measurements found there to be a difference of 0.71° between the mechanical axis and anatomic axis of the tibia. Similarly, Han and colleagues also found a difference in the tibial mechanical and anatomic axis in osteoarthritic knees.27 This deviation in the tibia mechanical and anatomic axes is most likely due to osteoarthritic wear and can be used with other input to determine the tibial cut. Using the MAKOplasty software tibial baseplate alignment is determined using the method described by Akagi.28 A recent systematic review evaluated the variety of methods used for tibial baseplate rotational alignment and found that the original Akagi line was the most accurate method and is reproducible.29 In our study we divided the anterior tibia and tubercle into sections (0 was medial to the tibial tubercle, 1 was the medial 1/3rd of the tubercle, 2 was middle 1/3 of the tubercle, 3 was the lateral 1/3rd of the tibial tubercle and 4 was lateral to the tibial tubercle). We then used the MAKOplasty software to determine where Akagi's line intersected the tibial tubercle on average. We found the line to on average intersect the medial 1/3rd of the tibial tubercle with fair agreement between raters which suggests that Akagi's line can be used to determine tibial component alignment using the MAKOplasty software.
Reliability analysis also exhibited significant agreement between raters for distal femoral valgus resection angle, distal femoral rotation angle, medial and lateral slope, and tibial tubercle alignment. This suggests that these measurements can be learned and reproduced using the software. In practice, a pharmaceutical representative calculates these measurements and produces a plan which is shared with the surgeon in the perioperative period. The plan is then fine-tuned intraoperatively with input from the surgeon. According to these results it appears that as long as representative is trained adequately no additional input is needed from the surgeon for preoperative planning.
Limitations to this study consist of small sample size, rater inexperience, and lack of intra-observer statistics. That said, our study design was an observational case series that simply supports current orthopaedic literature while utilizing MAKOPlasty software. As more robotic surgeries are performed larger studies are warranted to provide additional insights. Most surgeons do not commonly participate in the preoperative planning using the MAKOplasty. The main conclusions of our study is that using the stryker MAKOplasty protocol/software produces similar results to commonly calculated anatomic measurements used in planning for TKA with good reproducibility.
6. Description of study type
Observational Case Series.
Funding
No support received for this study.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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