1 Introduction

Kinematic alignment (KA) is an operative philosophy in total knee arthroplasty (TKA) that aims to restore natural knee kinematics by aligning knee components to the patient's unique pre-arthritic anatomy.^1–3 Unlike mechanical alignment (MA), which prioritizes aligning the mechanical axis of the femur and tibia, KA considers three functional axes that govern the movement of the patella and tibia relative to the femur.^4–6 By tailoring the alignment to each patient, KA seeks to replicate the natural movement of the knee, potentially improving functional outcomes and patient satisfaction (see Tables 3 and 4).

Proper prosthesis sizing in TKA is crucial for achieving optimal knee kinematics. Many systems are designed for the tibial component to match the femoral component size for optimized kinematics due to congruency of the femoral component and polyethylene geometry. Additionally, many systems are only compatible one size up or down due to nuances of the locking mechanism. Moreover, discrepancies in femoral and tibial component sizing have been linked to increased complication rates, including higher revision rates.⁷ Given the vast majority of components have been designed around a mechanical alignment philosophy, utilizing a kinematic philosophy with different alignment and rotation of the components may affect sizing and hence optimal kinematics.

The purpose of this study was to determine whether intrinsic differences between kinematic and mechanical alignment techniques lead to discrepancies in femoral and tibial prosthesis sizing for patients undergoing primary total knee arthroplasty.

2 Materials and methods

This retrospective study was conducted following institutional review board approval. Data was collected from patients who underwent primary total knee arthroplasty (TKA) performed by a single surgeon at a single institution between January 2017 and March 2024. Patients were identified through the institution's database using surgical procedure code CPT 27447. Inclusion criteria comprised adult patients (≥18 years) who underwent primary TKA. Exclusion criteria included revision TKA, complex procedures, salvage surgeries, reimplantation, and conversion procedures. Patients were categorized into two groups based on the alignment technique used during surgery, as determined by the surgeons' operative reports: mechanical alignment and kinematic alignment groups.

Operative data included procedure type (mechanical vs. kinematic alignment), operated knee (left vs. right), femoral and tibial component sizes, implant manufacturer, and insert specifications. Demographic data included age at the time of surgery, weight, height, body mass index (BMI), ethnicity, sex, and race. Femoral and tibial components of each TKA system were analyzed and stratified into three categories based on size discrepancies: femoral component larger than tibial component (Femoral > Tibial), femoral component equal to tibial component (Femoral = Tibial), and femoral component smaller than tibial component (Femoral < Tibial).

Predicted femoral and tibial sizes were calculated using published methodologies by Murphy et al. for predictions without preoperative radiographs. In accordance, the equation of Bhowmik-Stoker et al. was used, incorporating patient variables such as gender, age, height, weight, BMI, and ethnicity/race.^8,9 Femoral anteroposterior (AP) and tibial mediolateral (ML) dimensions were calculated in millimeters and compared with actual implanted component sizes. Only TKA systems using Attune and PFC Sigma prosthesis systems were included in the analysis, with sizing data obtained from publicly available resources provided by DePuy Synthes. Discrepancies between predicted and implanted component sizes were categorized as follows: predicted size larger than implanted component (Predicted > Actual), predicted size equal to implanted component (Predicted = Actual), and predicted size smaller than implanted component (Predicted < Actual).

3 Data analysis

Means and standard deviations (SD) summarized continuous variables; counts and percentages summarized categorical variables. Univariable Generalized Estimating Equations (GEE) were used to assess if Mechanical vs. Kinematic procedures were associated with sizing differences. Correlation structure accounted for multiple observations per patient. Other patients characteristics of age, BMI, sex, race, and ethnicity were also assessed. 2-sided P-values were deemed statistically significant. SAS 9.4 (Cary, NC) was used for statistical analyses.

4 Results

A total of 324 patients were included in the study, with a mean (SD) age of 66.2 (8.8) years. Of the cohort, 64 % were women, and 66 % identified as White. Among the patients, 178 underwent kinematically aligned TKA, while 146 underwent mechanically aligned TKA. The demographic characteristics between the two groups were comparable. The mean age for kinematic and mechanical alignment groups was 66.8 (8.5) and 66.5 (9.3) years, respectively (P = 0.179). The distribution of sex was also similar, with 37.1 % men in the kinematic group and 37.1 % men in the mechanical group (P = 0.597). Similarly, racial and ethnicity distributions showed no significant differences (P = 0.706 and 0.276, respectively). While average BMI was slightly higher in kinematic (34.0, SD 6.5) than mechanical (33.1, SD 6.3) alignment patients, there was no significant differences (P = 0.195). Further specifications are seen in Table 1.

Significant differences were observed in prosthesis sizing discrepancies between kinematic and mechanical alignment techniques (see Table 2). Mechanical alignment procedures were associated with a significantly higher incidence of femoral components being larger than tibial components, compared to kinematic alignment procedures (69.6 vs. 38.2 %, P < 0.001). In contrast, kinematic alignment showed a higher incidence of equal femoral and tibial sizes (51.1 vs. 28.8 %, P < 0.001) and femoral components smaller than tibial components (10.7 vs. 1.4 %, P < 0.001).

When comparing predicted femoral component sizes to actual implant sizes using the method proposed by Murphy et al., kinematic alignment procedures demonstrated a significantly higher incidence of predicted sizes being larger than actual sizes, compared to mechanical alignment procedures (68.7 vs. 53.6 %, P = 0.012). In contrast, mechanical alignment showed a higher proportion of predicted and actual femoral sizes being the same (22.9 vs. 16.3 %, P = 0.012) or predicted sizes being smaller than actual sizes (23.6 vs. 15.1 %, P = 0.012).

For tibial components, there was no significant difference between kinematic and mechanical alignment in terms of predicted versus actual sizes (P = 0.430). In the kinematic alignment group, 41.0 % of cases had predicted tibial sizes larger than the implanted sizes, 15.7 % had predicted and actual sizes matching, and 43.4 % had predicted sizes smaller than the implanted sizes. Similarly, in the mechanical alignment group, 43.6 % of cases had predicted sizes larger than actual sizes, 19.3 % matched predicted and actual sizes, and 37.1 % had predicted sizes smaller than the implanted sizes.

5 Discussion

The purpose of this study was to determine whether intrinsic differences between kinematic and mechanical alignment techniques lead to discrepancies in femoral and tibial prostheses sizing for patients undergoing primary Total Knee Arthroplasty (TKA). Kinematic alignment (KA) had a higher incidence of femoral components being smaller than tibial component (10.7 vs 1.4 %, P < 0.001). Similarly, KA had a higher incidence of femoral and tibial components being the same size (51.1 vs 28.8 %, P < 0.001). In contrast, mechanical alignment (MA) was associated with a higher incidence of femoral components being larger than tibial components (69.9 vs 38.2 %, P < 0.001). Predicated versus actual component size analysis further supported that KA results in smaller implants, with KA having a higher incidence of predicated femoral sizes being larger than actual sizes compared to MA (68.7 vs 53.6 %, P = 0.012). No significant differences were observed in predicated versus implanted tibial component sizes.

Campbell et al. studied the impact of femoral component position impacting the sizing in TKA. The authors found that changes in flexion and external rotation (ER) angles leads to changes in sizes. Specifically, at 0° flexion, reducing ER from 3° to 0° decreased femoral component size in 88 % of cases (10). Furthermore, increasing ER to 7° raised component size by 2.5 mm in 80 % of patients. Similarly, at 3° flexion and 3° ER, the component size decreased in 93 % of cases, and increasing ER or flexion from 3° to 7° increased the anterior-posterior height (APH) and component size. Kinematic alignment typically results in a more internally rotated femoral component when compared to MA.¹⁰ Our study's finding that KA, on average, results in a smaller femoral component than MA connects these previous studies.

As the femoral component tends to be more internally rotated with kinematic alignment, the tibial component also is more internally rotated to align with the flexion axis of the femur. As many tibial components are designed around a mechanical philosophy, internal rotation could theoretically require smaller sizes to avoid component overhang. However, this theoretical concern was no observed in this study.

Brar et al. examined the impact of KA and femoral component flexion on trochlear reach and patellar tracking, finding that flexion (5°, 10°, and 15°) in both MA and KA reduced proximal reach by 0.8mm/degree. Additionally, MA showed an average of 1 mm of lateral reach flexion deficit, while KA had 4 mm.¹¹ The authors argue this difference highlights the importance of designing a femoral component with a trochlea specifically shaped for KA. Borukhov et al. assessed KA and MA in TKA, finding KA better preserved trochlear sulcus orientation, with less deviation from the natural position (4.8° vs. 8.8°).¹² However, concerns remain over the medialization of the proximal trochlear angle with increasing valgus orientation of the femoral component and its potential impact on early patellar tracking. While neither alignment restored trochlear height in the proximal 80 %, KA better replicated it at the terminal arc point, whereas MA over-restored it due to requiring a larger femoral implant. This data, along with sizing information from our study, indicate the need to consider the principles of kinematic knee arthroplasty when designing femoral components.

6 Limitation

This study has several limitations. First, patient recruitment was conducted at a single institution, which may limit the generalizability of the findings to other institutions, states, or countries with varying patient populations. Second, the data was based on procedures performed by a single surgeon. While this consistency reduces variability, it may not accurately reflect the diverse decision-making processes and surgical techniques employed by other surgeons performing similar procedures.

Another limitation lies in the reliance on DePuy Synthes prosthesis systems for predicted value calculations. While this approach minimizes variability, it also restricts the study's applicability to the broad range of prosthesis types available in the market. Additionally, this study focused solely on primary total knee arthroplasty, with no analysis of postoperative follow-ups or radiographs. This assumption that the selected implant size was appropriate for each patient limits the ability verify proper implant fit. Furthermore, the absence of patient-reported outcome measures (PROMs) between alignment groups prevents assessment of whether these sizing differences impact patient outcomes. Although improved outcomes from optimized sizing are theoretically supported by current literature, direct evidence regarding discrepancies between alignment techniques remains lacking.

7 Conclusion

This study highlights prosthesis sizing discrepancies between kinematic and mechanical alignment in total knee arthroplasty. The significant variation in femoral sizing distributions emphasizes the importance of considering alignment techniques in component design with regards to sizing. With increased adoption of kinematic alignment in total knee arthroplasty, it is essential to recognize this fundamental difference and remain aware of other variables that arise between the two techniques.

Informed consent

Informed consent is not required for this work as this is not a case report and there are no clinical images obtain.

Author contribution

Jakub Gocal: conceptualization, methodology, validation, formal analysis, investigation, data curation, writing – original draft, writing – review and editing, visualization. Amy Wozniak: methodology, software, validation, formal analysis, writing – review and editing. Michael Murphy: methodology, software, validation, formal analysis, writing – review and editing. Nicholas Brown: conceptualization, methodology, validation, writing – original draft, writing – review and editing.

Ethical statement

Not applicable.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.