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Abstract

Background

Restoring native alignment or allowing slight residual varus is often considered optimal in medial unicompartmental knee arthroplasty (UKA). However, it remains unclear whether postoperative alignment remains stable over time and which preoperative factors contribute to varus progression.

Methods

We retrospectively reviewed 126 medial UKAs performed between 2017 and 2022 with a minimum follow-up of 3 years. Standing long-leg radiographs were obtained preoperatively, at 1–3 months postoperatively, and at final follow-up. Hip-knee-ankle (HKA), medial proximal tibial angle (MPTA), lateral distal femoral angle (LDFA), and tibial plateau tip-to-proximal tibial shaft (TPTPS) angle were measured. Change in alignment (ΔHKA) was defined as the difference between final and immediate postoperative HKA. Correlation and multivariate regression analyses were performed to identify predictors of ΔHKA, and clinical outcomes were analyzed relative to preoperative alignment.

Results

Mean HKA increased from 2.7° to 4.2° at final follow-up (p < 0.001). The proportion of outliers (HKA >5°) increased from 17% to 39%. ΔHKA was significantly correlated with preoperative HKA (r = 0.471), MPTA (r = −0.388), and TPTPS (r = 0.355) (all p < 0.001). In multivariate regression analysis, preoperative HKA, LDFA, and MPTA independently predicted ΔHKA (R² = 0.295, p < 0.001). No association was found between ΔHKA and clinical outcomes.

Conclusion

Varus progression may occur after medial UKA, particularly in patients with preoperative varus alignment and proximal tibial varus. Surgeons should consider these factors when determining intraoperative alignment targets to avoid excessive residual varus.

Keywords

unicompartmental knee arthroplasty alignment varus progression hip-knee-ankle angle tibial alignment

Background

Unicompartmental knee arthroplasty (UKA) provides a more natural joint sensation and enables faster recovery than total knee arthroplasty (TKA).^1,2 Furthermore, several studies have demonstrated favorable long-term clinical outcomes following UKA,^3,4 contributing to its increasing utilization in recent years.⁵

Achieving appropriate mechanical alignment is essential for the longevity of medial-compartment UKA. Restoration of the native pre-arthritic alignment has been associated with greater implant survivorship and improved patient-reported outcomes.⁶ Undercorrection leading to residual varus alignment may increase medial compartment loading, potentially causing early polyethylene wear and implant failure.⁷ Conversely, overcorrection into valgus alignment may overload the lateral compartment, accelerating the development of lateral compartment osteoarthritis.^8,9 Therefore, achieving alignment close to the patient’s native pre-arthritic state is generally considered an acceptable and beneficial strategy in medial UKA.^6,10 In the absence of pre-arthritic alignment information, mild-to-moderate varus alignment following medial UKA and moderate valgus alignment after lateral UKA have been reported to yield the most favorable outcomes.^11–13

Determining the optimal alignment target in UKA remains challenging. One difficulty arises from the wide variability in lower limb alignment phenotypes among individuals, which complicates efforts to establish a standardized alignment goal.¹⁴ Moreover, the alignment achieved immediately after surgery may not be maintained during follow-up. Although restoration of the native pre-arthritic alignment or slight undercorrection of varus deformity is generally considered acceptable in medial UKA, concerns persist regarding progressive varus drift over time, which could compromise implant longevity.

Previous studies on total knee arthroplasty (TKA) have identified proximal tibial varus anatomy and postoperative varus alignment as risk factors for long-term progressive malalignment.¹⁵ Furthermore, greater deviations from neutral alignment in the early postoperative period have been associated with more pronounced mechanical axis changes during follow-up.¹⁶ However, the influence of preoperative proximal tibial varus anatomy and postoperative varus alignment on progressive varus change after UKA has not been fully clarified.

The purpose of this study was to investigate postoperative changes in limb alignment following medial UKA and to identify factors associated with such changes. We hypothesized that varus progression would occur during mid-term follow-up and that this progression would correlate with preoperative proximal tibial morphology and varus limb alignment.

Materials and methods

Patient and data collection

This study was approved by the Institutional Review Board (IRB) of the authors’ institution. A retrospective review of patients who underwent medial UKA between January 2017 and December 2022 was performed. The indications for UKA included patients aged over 50 years with isolated medial compartment osteoarthritis, coronal plane deformity <15°, flexion contracture <15°, knee flexion >90°, intact lateral compartment cartilage, and an intact anterior cruciate ligament (ACL), all confirmed by magnetic resonance imaging (MRI). Patients were included if they underwent UKA for medial knee pain due to medial compartment osteoarthritis or osteonecrosis and had a minimum postoperative follow-up of 3 years. Patients with additional surgery for postoperative complications or with postoperative flexion contracture >5° were excluded to ensure reliable radiographic assessment. No patients had a history of infection, inflammatory arthritis, or bicompartmental arthroplasty.

Radiographic assessment

Radiographic examinations, including standing long-leg radiographs, were performed preoperatively in all patients. Standing long-leg radiographs were routinely obtained at 1, 3, 6, and 12 months postoperatively, and annually thereafter. For radiographic assessment, preoperative, immediate postoperative (1–3 months after surgery), and final follow-up (≥3 years postoperatively) radiographs were analyzed. Preoperative radiographs were acquired within 1 month before surgery.

The hip-knee-ankle (HKA) angle, medial proximal tibial angle (MPTA), lateral distal femoral angle (LDFA), and tibial plateau tip-to-proximal tibial shaft (TPTPS) angle were measured on standing long-leg radiographs.

The HKA angle was defined as the acute angle between the line connecting the hip center to the intercondylar notch of the distal femur and the line connecting the tibial intercondylar eminence to the talar center. Varus deformity of the proximal tibia was assessed using the MPTA, defined as the angle between the tibial mechanical axis and the tibial joint line. The LDFA was defined as the lateral angle between the femoral mechanical axis and the distal femoral joint line. To evaluate the preoperative morphology of the proximal tibia, the TPTPS angle was measured as the angle formed between a line drawn from the tip of the tibial plateau to the midpoint of the tibial shaft, located 7.5 cm distal to the plateau, and the proximal tibial shaft line, following the method described by a previous study (Figure 1).¹⁷

Figure 1.

Preoperative standing long-leg radiograph showing the measurement of the tibial plateau tip–to–proximal tibial shaft (TPTPS) angle.

Because of the radiographic contrast between the metallic femoral and tibial components on the medial side and the remaining cartilage and meniscus on the lateral side, postoperative measurements of MPTA and LDFA were modified.¹⁸ The postoperative MPTA was defined as the angle between a line extending from the distal end of the femoral component to the center of the lateral joint space and the tibial mechanical axis. The postoperative LDFA was defined as the lateral angle between a line drawn from the distal end of the femoral component to the center of the lateral joint space and the femoral mechanical axis (Figure 2). Because both the medial femoral and tibial cartilage, along with the subchondral bone, are resected during UKA, the distal end of the medial femoral component was used as the reference point for both measurements.

Figure 2.

(a) Preoperative standing long-leg radiograph showing measurement of the hip–knee–ankle (HKA) angle, lateral distal femoral angle (LDFA), and medial proximal tibial angle (MPTA). (b) Postoperative standing long-leg radiograph showing measurement of the HKA angle, LDFA, and MPTA.

Changes in the HKA angle during follow-up were compared with the immediate postoperative value. The variation in HKA (ΔHKA) was calculated as the difference between final follow-up and immediate postoperative values. In accordance with previous studies, patients with a postoperative HKA angle greater than 5° were classified as outliers.¹⁹ The proportion of outliers at each time point was compared using the chi-squared test. Pearson’s correlation and simple linear regression analyses were performed to identify associations between ΔHKA and factors such as BMI, preoperative and immediate postoperative HKA, TPTPS, LDFA, and MPTA angles. Variables showing significant correlations in the simple linear regression analysis were included in a multiple linear regression model.

Clinical assessment

All patients were evaluated preoperatively, at 6 and 12 months postoperatively, and annually thereafter. Clinical outcomes were assessed using the Knee Society Score (KSS), the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and the Forgotten Joint Score (FJS-12). Final follow-up outcomes were analyzed to evaluate correlations with preoperative tibial varus anatomy and varus alignment parameters, including the hip-knee-ankle (HKA) angle, medial proximal tibial angle (MPTA), and change in HKA (ΔHKA).

Surgical technique

All procedures were performed using fixed-bearing unicompartmental knee arthroplasty (UKA) systems. The implants included the Unicompartmental Persona® Partial Knee, Zimmer Uni®, and Zimmer® Unicompartmental High Flex Knee (ZUK) (Zimmer Biomet, Warsaw, IN, USA), as well as the Corin Uniglide® Unicompartmental Knee System (Corin Group, Tampa, FL, USA). The Zimmer fixed-bearing system was used in 81 knees, and other systems in 46 knees. All surgeries were performed by two experienced orthopedic surgeons using conventional instruments according to the manufacturer’s surgical protocol, aiming to achieve neutral to slight varus alignment. A midline skin incision and a parapatellar arthrotomy were used in all patients. A limited medial collateral ligament (MCL) release below the joint line was performed when necessary. The proximal tibial cut was made perpendicular to the tibial mechanical axis with a posterior slope of 5°, using an extramedullary guide while preserving the anterior cruciate ligament (ACL). The distal femoral cut was made parallel to the tibial cut, maintaining an appropriate flexion-extension gap. Osteophytes were removed, and trial components of appropriate size were inserted. The extension and flexion gaps were checked using a 2-mm tensor block. When the medial gap was excessively tight, additional deep MCL release was performed with a pie-crusting technique using an 18-gauge needle. All components were implanted with bone cement after confirming balanced soft tissue tension and alignment.

Data analysis

As in previous studies, two independent observers measured the HKA angle, LDFA, MPTA, and TPTPS angle to evaluate the reproducibility of radiographic measurements.¹⁵ Each observer performed two measurements with a 4-week interval. Intra- and inter-observer reliabilities were assessed using the intraclass correlation coefficient (ICC). Because the reliability was high, the measurements obtained by a single observer were used for subsequent analyses.

All data were expressed as mean ± standard deviation. Statistical analyses were performed using SPSS software (version 20.0; IBM, Armonk, NY, USA). A p-value <0.05 was considered statistically significant. Post hoc power analysis was performed using G*Power version 3.1.21 to assess the adequacy of the sample size for Pearson’s correlation and multiple linear regression analyses. Post hoc power for Pearson’s correlation was 0.97 (medium effect size = 0.3, α = 0.05, total sample size = 127), and post hoc power for multiple linear regression was 1.00 (R² = 0.295, α = 0.05, total sample size = 127, number of predictors = 4).

Results

A total of 145 consecutive primary medial unicompartmental knee arthroplasties (UKAs) were performed in 126 patients between 2017 and 2022 and analyzed retrospectively. The mean follow-up was 4.9 ± 1.5 years (range, 3.0-8.3). Patients with a history of postoperative periprosthetic fracture (5 patients) or postoperative flexion contracture >5° (3 patients) were excluded. 11 patients were lost to follow-up. Consequently, 126 UKAs in 106 patients with varus osteoarthritis were included in the final analysis. The patients had a mean age of 64.9 ± 7.2 years (range, 51–82) and a mean preoperative coronal alignment of 6.9° ± 2.9° (range, 0.3°–12.5°) based on the hip-knee-ankle (HKA) angle.

The mean HKA angle showed a significant increase between the immediate postoperative and mid-term postoperative evaluations (2.7° ± 2.4° vs 4.2° ± 2.8°, p < 0.001). The incidence of outliers (postoperative HKA >5°) was 22 of 126 knees (17%) immediately after surgery and increased to 49 of 126 knees (39%) at final follow-up, representing a statistically significant difference between the two time points (p < 0.001). The intraclass correlation coefficients (ICCs) for intra- and inter-observer reliability of TPTPS, LDFA, MPTA, and HKA measurements all exceeded 0.8, ranging from 0.83 to 0.94, indicating excellent reproducibility.

The preoperative HKA angle (r = 0.471, p < 0.001), preoperative MPTA (r = −0.388, p < 0.001), and preoperative TPTPS angle (r = 0.355, p < 0.001) showed moderate correlations with ΔHKA. The Pearson correlation coefficients for each potential predictor of ΔHKA are summarized in Table 1.

Table 1.

The mean values of radiographic parameters and their correlations with ΔHKA.

Variable	Mean ± SD (range)	Pearson correlation coefficient	p-value
BMI (kg/m²)	26.9 ± 3.2 (18.8 - 35.1)	−0.161	0.071
Preoperative TPTPS angle (°)	5.2±2.2 (0.5 - 9.5)	0.355	< 0.001
Preoperative LDFA (°)	88.0 ± 1.4 (84.4 - 90.3)	0.191	0.031
Preoperative MPTA (°)	84.9 ± 2.2 (81.0 - 90.0)	−0.388	< 0.001
Preoperative HKA angle (°)	7.0 ± 2.8 (0.9 - 12.0)	0.471	< 0.001
Immediate postoperative LDFA (°)	88.2 ± 2.4 (82.6 - 93.6)	−0.111	0.212
Immediate postoperative MPTA (°)	85.4 ± 2.3 (80.9 - 91.5)	−0.036	0.684
Immediate postoperative HKA angle (°)	2.7 ± 2.4 (-0.8 - 9.9)	0.162	0.069

BMI, body mass index; TPTPS, tibial plateau tip-to-proximal tibial shaft; LDFA, mechanical lateral distal femoral angle; MPTA, medial proximal tibial angle; HKA, mechanical hip-knee-ankle angle.

Simple linear regression analyses revealed that ΔHKA was positively correlated with the preoperative HKA angle (R² = 0.222, p < 0.001), preoperative MPTA (R² = 0.151, p < 0.001), preoperative LDFA (R² = 0.037, p = 0.031), and preoperative TPTPS angle (R² = 0.126, p < 0.001) (Figure 3). No significant correlations were found between ΔHKA and immediate postoperative HKA, MPTA, or LDFA angles.

Figure 3.

Scatter plots showing correlations between the change in the hip–knee–ankle (HKA) angle (ΔHKA) and (a) preoperative HKA, (b) medial proximal tibial angle (MPTA), (c) lateral distal femoral angle (LDFA), and (d) tibial plateau tip–to–proximal tibial shaft (TPTPS) angle.

Multiple linear regression analysis demonstrated that preoperative HKA, LDFA, and MPTA remained significant independent predictors of ΔHKA (overall model: F = 9.941, p < 0.001; R² = 0.295). The preoperative TPTPS angle was excluded from the stepwise regression model. The detailed results of the multiple regression analysis are shown in Table 2.

Table 2.

Multiple linear regression model for ΔHKA.

	β coefficient	t-value	p-value	VIF
Preoperative HKA angle (°)	0.155	3.28	0.001	1.876
Preoperative LDFA (°)	0.240	3.34	0.001	1.074
Preoperative MPTA (°)	−0.128	−2.06	0.042	1.974
Excluded variable
Preoperative TPTPS angle (°)	−0.326	−0.988	0.325	19.031

VIF, Variance Inflation Factor; HKA, mechanical hip-knee-ankle angle; LDFA, mechanical lateral distal femoral angle; MPTA, medial proximal tibial angle; TPTPS, tibial plateau tip-to-proximal tibial shaft.

Correlations between postoperative patient-reported outcome measures (PROMs) and preoperative alignment parameters were also assessed. The FJS-12 score showed no significant correlation with preoperative HKA (r = −0.07, p = 0.436) or MPTA (r = 0.044, p = 0.625). Postoperative KSS demonstrated no significant correlation with either preoperative HKA (r = −0.055, p = 0.536) or MPTA (r = −0.078, p = 0.382). No significant correlations were observed between the postoperative WOMAC total score and preoperative HKA (r = 0.136, p = 0.128) or MPTA (r = −0.135, p = 0.129). Finally, ΔHKA was not significantly correlated with postoperative KSS (r = −0.066, p = 0.461) or WOMAC score (r = 0.045, p = 0.613).

Discussion

The most important finding of this study was that the preoperative hip-knee-ankle (HKA) angle and proximal tibial varus anatomy significantly affected postoperative varus progression after medial unicompartmental knee arthroplasty (UKA). Changes in postoperative HKA were significantly correlated with preoperative HKA, medial proximal tibial angle (MPTA), and lateral distal femoral angle (LDFA). Patients with severe preoperative varus deformity tended to experience progressive varus alignment over time, even when substantial correction was achieved immediately after surgery. This tendency should therefore be carefully considered when performing UKA in patients with pronounced varus alignment, and further long-term studies are warranted to evaluate the clinical implications of this progression.

Recent trends in UKA have emphasized kinematic alignment or mild residual varus correction to replicate a patient’s native alignment and optimize function. Several studies have reported that preserving pre-arthritic alignment results in more natural joint kinematics and higher patient satisfaction, especially in those with constitutional varus alignment.^6,18,20 However, the present study demonstrated that residual varus alignment achieved immediately after surgery may not remain stable over time. Patients with proximal tibial varus morphology or preoperative varus limb alignment showed significant varus progression during mid-term follow-up. These findings suggest that caution is needed when applying kinematic alignment or undercorrection strategies in UKA. Importantly, the present findings do not imply long-term mechanical failure or implant survival but rather highlight the potential for alignment drift during the mid-term period. Because alignment changes were assessed between the immediate postoperative period and final follow-up, the present study was not designed to identify a specific temporal cutoff point at which HKA changes begin. The potential for progressive varus malalignment and its long-term effects on implant longevity and load distribution should be carefully considered, particularly in patients with pronounced proximal tibial varus anatomy.

Several previous studies have examined the impact of preoperative bone morphology on coronal alignment after UKA. Park et al.²¹ reported that proximal tibial varus with an MPTA ≤86° increased joint line obliquity (JLO) and joint space malalignment (JSM) but did not affect clinical outcomes after a minimum 5-years follow-up. Erlichman et al.²² similarly found that patients with proximal tibial varus anatomy were not at increased risk for tibial component-related or all-cause failure in medial UKA. These results are consistent with our findings that preoperative varus alignment and proximal tibial varus morphology were not associated with clinical outcome scores. Unlike previous studies focusing primarily on survivorship or patient outcomes, our study quantified the correlation coefficients between anatomical parameters and alignment change, providing a more detailed understanding of alignment progression. By quantifying both preoperative tibial anatomy and alignment changes over a 3-years follow-up period, we demonstrated that postoperative limb alignment after medial UKA may continue to drift toward varus over time.

Our findings are also in line with previous reports on total knee arthroplasty (TKA). Kuroda et al.¹⁵ observed progressive varus alignment 10 years after TKA, associated with higher preoperative tibial plateau tip-to-proximal tibial shaft (TPTPS) angles, higher postoperative LDFA and HKA angles, and lower postoperative MPTA. Similarly, Teeter et al.²³ reported that increased varus positioning of the tibial component led to greater implant migration and lateral compartment lift-off over 10 years. However, few studies have explored varus progression in relation to preoperative anatomical parameters and limb alignment after UKA. The present study identified that postoperative varus progression was significantly related to preoperative varus alignment and proximal tibial varus anatomy.

A potential mechanism underlying this progression may be the preserved soft tissue structures and bone stock in UKA, which allow ongoing adaptive remodeling or gradual medial laxity, potentially shifting the mechanical axis medially. Unlike TKA, postoperative limb alignment after UKA may be more strongly influenced by preoperative alignment and soft tissue balance, with native tibial morphology playing a greater role in long-term stability.

In this study, immediate postoperative MPTA and LDFA were not significantly associated with alignment progression. This may be due to technical limitations in postoperative angle measurement, as the medial compartment is replaced by implants while the lateral compartment remains native. Conventional bony landmarks used for preoperative measurements may not be directly applicable postoperatively. Previous studies on TKA have suggested that immediate postoperative varus alignment can predict long-term varus progression.¹⁵ In contrast, our findings indicate that immediate postoperative varus anatomy, such as low MPTA or high LDFA, does not necessarily predict varus progression in UKA. The modified measurement method used in our study may also underestimate postoperative MPTA and LDFA values, which could explain the weaker correlation observed between immediate postoperative angles and varus progression.

The observed postoperative varus progression in this study should be interpreted with caution. Although progressive varus alignment was evident during mid-term follow-up, it was not associated with inferior patient-reported outcome measures, indicating that such radiographic changes do not necessarily reflect early implant failure. Varus progression after medial UKA may represent a relatively benign radiographic or biomechanical adaptation in the mid-term. Importantly, no cases of revision surgery or clinical implant failure were observed during the follow-up period, and changes in the HKA angle alone did not indicate a failing implant. However, because progressive varus alignment may alter load distribution and potentially influence implant longevity over time, its long-term clinical significance remains uncertain and warrants further investigation with extended follow-up.

In addition to preoperative anatomical factors, surgical technique–related variables may also influence postoperative alignment behavior. Factors such as component alignment, navigation- or robotic-assisted surgery, and alignment restoration strategies have been reported to affect coronal alignment after UKA.^24–27 Although these variables were not included in the multivariate regression analysis in the present study because of the study design and limited subgroup sizes, their potential influence on postoperative varus progression should be acknowledged when interpreting the results. Future studies incorporating these surgical technique–related factors may help further clarify their relative contributions to alignment changes after UKA.

Our results have several clinical implications. First, careful preoperative evaluation of tibial morphology—particularly MPTA—may help identify patients at higher risk for postoperative varus progression. Second, surgeons should be aware that even when acceptable alignment is achieved immediately after surgery, patients with severe preoperative varus deformity and varus tibial morphology may still experience gradual varus progression over time. These findings support considering alignment targets closer to neutral rather than residual varus correction in high-risk patients. However, overcorrection into valgus alignment should be avoided due to the risk of lateral compartment overload.^8,9

Limitations

This study has several limitations. First, it was a retrospective, single-center analysis with a minimum follow-up of 3 years, which is sufficient for mid-term radiographic assessment but insufficient to determine implant survivorship or long-term mechanical drift. Therefore, the findings should not be interpreted as having long-term predictive value. In particular, the absence of revision or failure events precluded analysis of whether changes in HKA angle could predict prosthesis failure. Second, although radiographs were obtained at predefined postoperative intervals, the retrospective design and variability in follow-up timing precluded identification of a specific temporal cutoff point at which HKA changes begin. Prospective longitudinal studies with fixed and frequent interval assessments are needed to better characterize the timing and pattern of postoperative alignment changes. Third, surgeries were performed by two different surgeons using two different instrumentation systems. Although surgical techniques were nearly identical, inter-surgeon and instrument variability may have influenced the radiographic outcomes. Fourth, only fixed-bearing implants were included, and thus the findings may not apply to mobile-bearing UKA. Fifth, radiographic measurements after UKA may be subject to potential measurement bias because conventional bony landmarks are partially obscured by metallic components. Although a modified measurement method was used and high intra- and inter-observer reliability was confirmed, inter-method comparability with preoperative measurements may be limited, and subtle measurement errors cannot be completely excluded. Lastly, radiographic progression (ΔHKA) was not correlated with functional outcomes. Therefore, whether varus progression translates into clinical deterioration remains uncertain. Despite these limitations, this study is meaningful as it provides quantitative mid-term evidence of varus progression after medial UKA in patients with preoperative varus tibial morphology, an area that has been underexplored.

Conclusion

Varus progression may occur after medial UKA, particularly in patients with preoperative varus alignment and proximal tibial varus. Surgeons should consider this risk when determining intraoperative alignment targets to avoid excessive residual varus.

Footnotes

ORCID iDs

Ji Hyun Ahn

Dong-Wook Son

Ethical considerations

This study was approved by the Institutional Review Board (IRB) of Kangbuk Samsung Hospital (Approval No. KBSMC 2023-08-020).

Consent to participate

The requirement for informed consent was waived due to the retrospective design. All procedures involving human participants complied with the ethical standards of the institutional and national research committees and with the 1964 Helsinki Declaration and its later amendments.

Author Contributions

D.W.S. contributed to the study conception and design. J.H.A. and D.W.S. performed radiologic measurements. D.W.S. conducted the statistical analysis and drafted the manuscript. All authors read and approved the final version of the manuscript.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Appendix

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26.

Sun

Liu

Hou

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27.

Sava

Leica

Scala

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Preoperative proximal tibial varus and varus limb alignment contribute to mid-term varus progression after fixed-bearing medial unicompartmental knee arthroplasty

Abstract

Background

Methods

Results

Conclusion

Keywords

Background

Materials and methods

Patient and data collection

Radiographic assessment

Clinical assessment

Surgical technique

Data analysis

Results

Discussion

Limitations

Conclusion

Footnotes

ORCID iDs

Ethical considerations

Consent to participate

Author Contributions

Funding

Declaration of conflicting interests

Appendix

References