Most of the existing total knee arthroplasty (TKA) implants are designed to suit the knee anatomy of the Western population. Studies have shown that there are striking variations in knee morphology between the Asian and Western population. We conducted a case-control study to investigate the anthropometry of the Indian arthritic knees using three dimensional knee models and evaluated them morphologically against commercially available TKA implants. Measurements included the mediolateral width (ML), anteroposterior width (AP) and aspect ratio of femur and tibia. Indian females were found to have smaller dimensions of femur and tibia than Indian males and both had smaller aspect ratios than the Western population. Our study suggests that there is a morphological mismatch between existing implants and Indian arthritic knees. This may mean that Western implants have drawbacks when implanted in Indian patients.
The principal factors on which the long life and successful outcome of a total knee arthroplasty (TKA) implant depend are optimum bone resection and maximum coverage of the resected bone [
66 knee joints from living subjects were analyzed in this study. These subjects were patients suffering from bilateral primary osteoarthritis undergoing TKA. The knee joint data included fourteen male knees and fifty-two female knees. This data was collected preoperatively using Computed Tomography (CT) imaging. Patients having other periarticular pathologies such as tumours, rheumatoid arthritis and post-traumatic fracture were excluded from this study. Since both knees of all patients were affected by osteoarthritis, both left and right knees were included in the database.
For all subjects, a CT scan of the knee was obtained using a helical CT scanner (130 kV, 53 mA; Samsung Healthcare). The patients were informed about the procedure and a written approval was taken from them for the purpose of statistical study. During the scan, it was ensured that the subjects were supine with the knees in relaxed and extended position. This scanning procedure was performed to acquire CT slices of thickness 0.5mm with a resolution of 512 × 512 pixels. The raw data obtained from the scans was first copied in DICOM file format and then exported to a medical modelling software (Mimics 10.1, Materialise, Belgium). The images were segmented to construct 3D bone models of the knee. The approach mentioned above was similar to those reported in literature [
In order to accurately quantify the difference between Indian arthritic knees and commercially available TKA implants, a resection study was performed by simulating the surgical procedure on the solid models of the knee. Conventional surgical cuts performed on the knee during the TKA surgery were virtually simulated.
Once the 3D model of the femur was obtained, the distal surgical cut was virtually performed on the femur [
Once the 3D model of the tibia was obtained, the proximal surgical cut was virtually performed on the tibia [
Virtual surgical resections (a) Distal femur (b) Proximal tibia
The resected femur was oriented perpendicular to the transverse plane and all measurements were performed on this distal resection plane (
The resected tibia was oriented perpendicular to the transverse plane and all measurements were performed on this proximal resection plane (
Based on the guidance and feedback of surgeons, a fixed implant selection procedure was decided which was commonly followed by knee arthroplasty surgeons for conventional implants. As mentioned in literature, clinical consequences of overhangs in TKA are disastrous [
In case of the femoral component, the fML is the deciding dimension i.e. the size of the implant is selected such that the ML dimension of the implant closely matches to that of the subject but is less than or equal to the fML of the subject. This is done to ensure no overhangs in ML dimension.
In case of the tibial component, the size of the implant is selected such that the ML dimension of the implant closely matches to that of the subject but is less than or equal to the tML of the subject. Thus, any possible overhang in ML dimension is prevented. However, while doing so, if there is an overhang in AP dimension, then the size of the implant is selected such that the AP dimension of the implant closely matches to that of the subject but is less than or equal to the tAP of the subject. Thus, overhang is prevented in the AP dimension, although a compromised underhang occurs in ML dimension.
In the statistical analysis, best-fit lines were calculated with use of least squares regression method. The data was summarized as the mean and standard deviation (mean ± SD). Data analysis of the Indian knees was performed by using the Student t-test. Welch’s t-test was used to compare data with dissimilar number of population samples. Values for p < 0.05 were regarded as statistically significant.
The average age for males was greater than the average age of females (73.6 ± 8.4 years, range 64 - 82 years for
Measurements performed on resected planes (a) Distal femur (b) Proximal tibia
males vs. 66.2 ± 8.2 years, range 52 - 80 years for females, p = 0.07).
Average femoral mediolateral length (fML) was 68.3 ± 3.9 mm and average femoral anteroposterior length (fAP) was 62.7 ± 4.8 mm. Average femoral aspect ratio (fML/fAP) was 1.09 ± 0.05. Males had larger values of fML and fAP as compared to females (p < 0.01).
A positive correlation was observed between fAP and fML for both males and females. In case of the femoral aspect ratio (fR), no statistically significant difference was found between males and females (p = 0.998). Femoral measurements for the Indian population have been compared across genders in
The average tibial mediolateral length (tML) was 73.8 ± 4.7 mm and average tibialanteroposterior length was 51.2 ± 4.3 mm. The average tibial aspect ratio (tML/tAP) was 1.45 ± 0.08. Males had larger values of tML and tAP as compared to females (p < 0.01). A positive correlation was observed between tAP and tML for both the genders. In case of the tibial aspect ratio (tR), no statistically significant difference was found between males and females (p = 0.588). Tibial measurements for the Indian population have been compared across genders in
To compare the Indian knees morphologically with other ethnic groups, the anthropometric measurements of knees of various ethnicities were retrieved from literature. The aspect ratios were defined as mentioned above in order to make our data coherent with the anthropometric data of other ethnic groups. The data of the distal femur of Indian population was compared with that of Chinese, Caucasian, Japanese, Korean and American population as shown in
The Indian knees were compared to the following commercially available conventional implants – JOURNEY Knee (Smith & Nephew, London), PFC Sigma Knee System (DePuy, NJ, USA), ADVANCE Knee System (Wright Medical Technology Inc., TN, USA), Triathlon Total Knee (Stryker Corp., MI, USA), NexGen LPS Flex Mobile Knee System (Zimmer Holdings, IN, USA) and Scorpio NRG Knee System (Stryker Corp., MI, USA).
In case of the femoral components, the various implant sizes covered the entire range of the Indian population (
In case of the tibial components, the various implant sizes covered the entire range of the Indian population
. Average values of the Indian femur and tibia morphology measurement.
Parameters | Combined | Male | Female | p a |
---|---|---|---|---|
Femoral mediolateral (fML) | 68.3 ± 3.9 | 71.5 ± 2.5 | 65.1 ± 3.1 | <0.01 |
Fem.anteroposterior (fAP) | 62.7 ± 4.8 | 65.6 ± 3.8 | 59.8 ± 4.3 | <0.01 |
Femoral aspect ratio (fR = fML/fAP) | 1.09 ± 0.05 | 1.09 ± 0.04 | 1.09 ± 0.05 | 0.998 |
Tibialmediolateral (tML) | 73.8 ± 4.8 | 77.5 ± 4.0 | 70.0 ± 3.4 | <0.01 |
Tibialanteroposterior (tAP) | 51.2 ± 4.3 | 53.9 ± 2.8 | 48.5 ± 3.9 | <0.01 |
Tibial aspect ratio (tR = tML/tAP) | 1.45 ± 0.08 | 1.44 ± 0.05 | 1.45 ± 0.09 | 0.588 |
a. The p-values are calculated for comparison across genders.
. Summary of the measurements of distal femur across various ethnic groups.
Author | Ethnicity | fMLa | fAPa | fR (fML/fAP)a |
---|---|---|---|---|
Our study | Indian | 71.5 ± 2.5(M) 65.1 ± 3.1(F) 68.3 ± 3.9(C) | 65.6 ± 3.8(M) 59.8 ± 4.3(F) 62.7 ± 4.8(C) | 1.09 ± 0.04(M) 1.09 ± 0.05(F) 1.09 ± 0.05(C) |
Yue et al. [15] | Chinese | 82.6 ± 3.6(M) 72.8 ± 2.6(F) | 65.0 ± 2.8(M) 58.8 ± 2.5(F) | 1.27 ± 0.03(M) 1.24 ± 0.04(F) |
Yue et al. [15] | Caucasian | 86.0 ± 5.6(M) 76.4 ± 4.0(F) | 67.5 ± 3.6(M) 59.7 ± 2.6(F) | 1.28 ± 0.07(M) 1.28 ± 0.06(F) |
Berger et al. [16] | American | 85.6 ± 5.1(M) 75.4 ± 2.3(F) | 68.1 ± 4.6(M) 60.2 ± 2.0(F) | |
Mensch et al. [17] | American | 82.1 ± 4.7(M) 69.9 ± 2.6(F) 76.8 ± 7.2(C) | ||
Wang et al. [18] | Chinese | 63.3 ± 4.7(C) | ||
Griffin et al. [19] | American | 84.1 ± 4.4(M) 74.1 ± 4.6(F) 78.0 ± 6.7(C) | ||
Urabe et al. [5] | Japanese | 70.6 ± 4.5(C) | ||
Chin et al. [20] | American | 71.6(C) | 57.3(C) | |
Lee et al. [13] | Korean | 75(C) | 57(C) | |
Ho et al. [4] | Chinese | 70.2 ± 5.4(C) | 63.7 ± 5.1(C) | 1.09 ± 0.06(C) |
Cheng et al. [10] | Chinese | 74.4 ± 2.9(M) 66.8 ± 3.1(F) 71.0 ± 3.0(C) | 66.6 ± 2.4(M) 61.0 ± 2.7(F) 64.1 ± 2.7(C) | 1.12 ± 0.03(M) 1.09 ± 0.04(F) 1.11 ± 0.03(C) |
a. Data presented as mean ± SD in mm. Abbr.—fML: femoral mediolateral, fAP: femoral anteroposterior, fR: femoral aspect ratio.
(
Although the graphs of ML vs. AP, for both femur and tibia, suggested that Western implants covered the demographic population, the mismatch of foreign implants on Indian knees needed to be closely observed. To correctly quantify implant errors, every patient’s femur and tibia was virtually fitted with an optimal implant component of each implant manufacturer, according to the procedure mentioned previously.
. Summary of the measurements of proximal tibia across various ethnic groups.
Author | Ethnicity | fMLa | fAPa | fR (fML/fAP)a |
---|---|---|---|---|
Our study | Indian | 77.5 ± 4.0(M) 70.0 ± 3.4(F) 73.8 ± 4.8(C) | 53.9 ± 2.8(M) 48.5 ± 3.9(F) 51.2 ± 4.3(C) | 1.44 ± 0.05(M) 1.45 ± 0.09(F) 1.45 ± 0.08(C) |
Cheng et al. [10] | Chinese | 76.4 ± 2.8(M) 68.8 ± 4.6(F) 73.0 ± 4.6(C) | 51.3 ± 2.0(M) 45.7 ± 1.9(F) 48.8 ± 3.4(C) | 1.49 ± 0.06(M) 1.51 ± 0.06(F) 1.49 ± 0.05(C) |
Kwak et al. [3] | Korean | 76.1 ± 4.0(M) 67.6 ± 3.1(F) 71.9 ± 5.6(C) | 48.2 ± 3.3(M) 43.2 ± 2.3(F) 45.7 ± 3.8(C) | 1.58 (M) 1.56 (F) 1.57 (C) |
Mensch et al. [17] | American | 80.3 ± 3.7(M) 70.1 ± 2.8(F) | 48.9 ± 2.3(M) 42.1 ± 1.7(F) | |
Westrich et al. [6] | American | 42.9 ± 6.8(C) | ||
Uehara et al. [7] | Japanese | 83.0 ± 6.2(M) 71.7 ± 4.0(F) 74.3 ± 6.6(C) | 53.8 ± 6.6(M) 46.6 ± 3.6(F) 48.3 ± 5.4(C) |
a. Data presented as mean ± SD. Units in mm. Abbr.—tML: tibial mediolateral, tAP: tibialanteroposterior, tR: tibial aspect ratio.
Comparison with conventional implants (a) fML vs. fAP (b) tML vs. tAP (c) fMLfAP vs. fAP (d) tMLtAP vs. tAP
In case of the femur, fML being the deciding dimension, errors in this dimension were minimized by selecting an optimal implant size. Thus, the overhangs in this dimension were negligible; however, due to fixed sizes of implants, underhangs up to 5 mm were observed (
In case of the tibia, the selection criterion only involved minimizing overhangs i.e. any one of tML and tAP could be the deciding dimension. Hence, two sets of studies were analyzed—one with tML as the deciding dimension and the other with tAP as the deciding dimension.
In the first case, with tML being the deciding dimension, errors in this dimension were minimized by selecting an optimal implant size. Thus, it was ensured that there are no overhangs in this dimension; however, due to fixed sizes of implants, underhangs up to 7 mm were observed (
In the second case, with tAP being the deciding dimension, errors in this dimension were minimized by selecting an optimal implant size. Thus, overhangs in this dimension were negligible; however, due to fixed sizes of implants, underhangs up to 4 mm were observed (
Total knee replacement, being a precise surgical procedure, shape match between the prosthesis and the resected surface of the knee is a vital factor for long-term success [
Error in optimally selected femoral components (a) fML error (b) fAP error (c) fR error
Error in optimal tibial components selected with ML dimension as base (a) Error in tML (b) Error in tAP (c) Error in tR
Error in optimal tibial components selected with AP dimension as base (a) Error in tAP (b) Error in tML (c) Error in tR
implants are required to cover maximum resected surface in order to achieve a successful outcome [
Morphological measurements of the ML and AP dimensions revealed that the Indian female has a smaller femur and tibia than the Indian male, and both have smaller values than the Western population. Probably owing to differences in the imaging technique, some minor differences between other Asian populations were also observed.
The Indian knees were compared to Western implants which were commonly available in the Indian market and used by surgeons for TKA of Indian patients. The femoral data of the Indian population showed a decreasing aspect ratio (fML/fAP) as the fAP dimension increased. In case of the Indian tibia, the aspect ratio (tML/tAP) decreased as the tAP increased. In contrast to these observations in the aspect ratios of Indian patients, variations in aspect ratios of most implants were insignificant. For both the femur and tibia, it was found that the ML dimension was undersized with smaller AP and overhang was observed for larger AP dimensions. Thus our results suggest that Western implants, although suitable for the Western population, may not be suitable for Indian patients. To quantify the mismatch of the above mentioned foreign implants, error extremities signifying overhangs and underhangs were exploited.
A radiographic and cadaveric study, that evaluated sizing for knee prostheses, supported the idea that the required femoral designs component size should be based on the ML dimension of the femoral condyle [
One of the limitations of our analysis is the lack of morphologic data of healthy, non-arthritic knees. In this study, we chose osteoarthritic knees to study and compare the morphology of Indian knee with other ethnicities and foreign implants. However, for the Indian population, whether prosthesis should be designed according to the normal or osteoarthritic knee morphology needs further research. Moreover, we measured only one resected surface in the tibia and femur in this study since it involved comparison with conventional implants; however, in recent clinical procedures, the depth of the cut surface is determined according to the stage of the osteoarthritis present [
Our study establishes that Western implants may have drawbacks when implanted in Indian patients. Moreover, this study provides definite rationale for designing total knee prosthesis, especially a gender-specific design suitable for the Indian population. This study clearly shows that while designing the prostheses, the relationship between aspect ratio and anteroposterior dimension of the tibia and femur should also be taken into account apart from the relationships between mediolateral and anteroposterior dimensions.
The authors have declared that there is no conflict of interest associated with this study.
This work has been carried out at the OrthoCAD Lab set up with funding by the Office of the Principal Scientific Advisor to the Government of India, New Delhi. The extensive support, including CT imaging and data provided by the Department of Orthopaedics at Hiranandani Hospital, Mumbai is gratefully acknowledged.