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. 2020 Apr 6;478(7):1636–1644. doi: 10.1097/CORR.0000000000001260

Valgus Correctability and Meniscal Extrusion Were Associated With Alignment After Unicompartmental Knee Arthroplasty

Kyota Ishibashi 1,2,✉, Eiji Sasaki 1,2, Hironori Otsuka 1,2, Koyama Kazushige 1,2, Yuji Yamamoto 1,2, Yasuyuki Ishibashi 1,2,✉
PMCID: PMC7310375  PMID: 32324667

Abstract

Background

Appropriate postoperative lower limb alignment is one important element of a successful unicompartmental knee arthroplasty (UKA). To predict postoperative alignment, it is important to investigate the association between preoperative imaging evaluations and lower limb alignment after medial UKA.

Questions/purposes

(1) Do preoperative valgus stress radiographic and MRI measurements (% mechanical axis, hip-knee-ankle angle, medial meniscal extrusion distance, and osteophyte area at the medial femur and tibia) correlate with postoperative lower limb alignment after UKA; and (2) Can useful cutoffs be calculated in advance of surgery for those findings that were associated with coronal-plane overcorrection?

Methods

We retrospectively analyzed 125 patients with medial knee pain who underwent UKA from January 2012 to October 2018. Valgus stress radiography and MRI were performed routinely to assess the knee. Valgus stress radiography was obtained with the patient supine with the knee in full extension and a firm manual valgus force applied to the knee. Full-length weightbearing radiography was performed 3 months after surgery. There were 12% (15) of patients who did not undergo MRI, and 4% (five) of patients who did not receive the postoperative full-length weightbearing radiograph and they were excluded, leaving 84% (105) of patients available for analysis. There were 27 men and 78 women with a mean (range) age of 77 years ± 6 years (60 to 87). The preoperative diagnosis was medial osteoarthritis in 99 patients and osteonecrosis of the medial femoral condyle in six. To investigate the associations, we routinely measured % mechanical axis using radiography, and also measured the medial meniscal extrusion distance and osteophyte area at the medial femur and tibia using MRI after surgery. Medial meniscus extrusion distance was defined as the distance from the outermost edge of the medial meniscus to a line connecting the femoral and tibial cortices. From these parameters, postoperative alignment was estimated using regression and receiver operating characteristic curve analyses. Variables with p < 0.05 were included.

Results

The % mechanical axis on the valgus stress radiograph and medial meniscal extrusion distance were correlated with postoperative lower limb alignment after UKA (adjusted correlation coefficient 0.72; p < 0.001, adjusted correlation coefficient 0.2; p = 0.003, respectively). The estimated % mechanical axis on the postoperative weightbearing radiograph was equal to -0.27 + 0.86% (% mechanical axis on valgus stress radiograph) + 1.14 mm (medial meniscal extrusion distance). Using a cutoff point of 36%, the % mechanical axis on valgus stress radiograph was associated with overcorrection after UKA (area under the curve: 0.89; odds ratio 14 [95% CI 0.75 to 0.95]; p < 0.001, sensitivity 77.8%, specificity 80.9%).

Conclusions

The overcorrection of a varus knee on a valgus stress radiograph before UKA and the increased extrusion of the medial meniscus on preoperative MRI was associated with a greater likelihood of overcorrected alignment after UKA. Future studies should conduct long-term follow-up of malalignment patients to assess the possible complications.

Level of Evidence

Level III, diagnostic study.

Introduction

Unicompartmental knee arthroplasty (UKA) offers several potential advantages over TKA, including a knee that patients report feels more normal, perhaps because both cruciate ligaments are preserved [20, 23], and a shorter convalescence [19]. UKA is also associated in some series with excellent long-term clinical results [14,16-18, 24]. The percentage of UKA in the United States has increased dramatically [2]. However, malalignment and overcorrection of the mechanical axis from varus into valgus are important causes of premature revision after medial-compartment UKA [13, 28]. Obtaining appropriate postoperative lower-limb alignment is one of the key factors in the success of UKA. Although soft-tissue releases can help achieve ligament balance during TKA, many authors recommend against performing any release of the medial collateral ligament during medial UKA [22, 25] because doing so can result in excessive loading of the lateral compartment and the development of lateral-compartment arthritis [17, 28]. In contrast, neutral or slight under-correction may be tolerated [10].

The ability to overcorrect the mechanical axis of a knee before medial-compartment UKA may be associated with a higher likelihood of overcorrection during UKA; however, to our knowledge, this has not been studied. Furthermore, removal of the meniscus and osteophytes during UKA may overcorrect the mechanical axis. If this is true, it is concerning, because overcorrection into valgus has been associated with a higher risk of reoperation after UKA [28]. In addition, arthritis progression into the lateral compartment is a well-documented indication for premature revision after medial UKA [13].

A valgus stress radiograph may be performed to assess correctability before UKA [27, 34], and can be used to assess cartilage thickness in the lateral knee compartment [9, 31]. A valgus stress radiograph reflects the maximally corrected knee alignment before surgery in the supine position; however, the influences of osteophytes and the meniscus are unknown. Thus, there is a slight discrepancy between valgus stress radiograph findings and postoperative lower-limb alignment [15]. MRI may also be performed before UKA; however, whether these examinations are truly useful for planning is unclear.

We therefore asked: (1) Do preoperative valgus stress radiographic and MRI measures (% mechanical axis (%MA), hip-knee-ankle angle, medial meniscal extrusion distance, and osteophyte area at the medial femur and tibia) correlate with postoperative lower limb alignment after UKA? (2) Can useful cutoffs be calculated in advance of surgery on those findings that were associated with coronal-plane overcorrection?

Patients and Methods

We retrospectively analyzed 125 patients with medial knee pain who underwent medial UKA from January 2012 to October 2018. Valgus stress radiography and MRI were performed routinely to assess the knee. Full-length weightbearing radiography was performed 3 months after surgery. Nine percent (15) of patients did not receive MRI, and 4% (5) of patients did not receive the postoperative weightbearing radiograph and they were excluded, leaving 84% (105) patients who were eligible for analysis. The patients were followed for 34 ± 18 months (range 12 to 80). Since we only performed a clinical assessment of ROM and alignment in this study, we also included patients with a follow-up of less than 2 years. The study included 27 men and 78 women with a mean (range) age of 77 years ± 6 years (60 to 87), and a mean BMI of 28 ± 4 kg/m2 (Table 1). Fifty-six UKAs were performed on the right knee and 49 were performed on the left knee. The preoperative diagnosis was medial osteoarthritis in 99 patients and osteonecrosis of the medial femoral condyle in six. The indications for UKA were patients older than 65 years with severe knee pain of the medial compartment and disability in walking and activities of daily living. Other conditions were also indication criteria: an intact ACL, flexion contracture less than 15°, and an intact and lateral compartment including an uninjured discoid. Rheumatoid diseases, such as rheumatoid arthritis and psoriatic arthritis, patellofemoral osteoarthritis, and genu recurvatum were contraindications. The mean preoperative extension angle was -2 ± 3° and the mean preoperative flexion angle was 135 ± 7°. The mean postoperative extension angle was -1 ± 2° and the mean postoperative flexion angle was 136 ± 5° (Table 1). This study design was approved by the ethics committee of our institution, and all patients signed an informed consent document before surgery.

Table 1.

Preoperative patient demographic data

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Surgical Procedure

All UKAs were performed using the following surgical procedure. The components in this study were fixed-bearing (Unicompartmental Persona® partial knee and Zimmer Uni®; Zimmer Biomet, Warsaw, IN, USA). A standard midline incision was made, and the knee was exposed through a mid-vastus incision. Medial osteophytes were removed from the femur and tibia. A 2° varus tibial resection was performed perpendicular to the long axis with an extra-medullary guidance system. A spacer block was then inserted into the joint space at full extension. A distal femoral cut with the knee in extension was made, ensuring that the proximal tibial and distal femoral cuts were parallel. The posterior femoral resection then created a flexion gap equal to the extension gap. The thickness of the polyethylene was adjusted to ensure a well-balanced knee capable of full extension with 2-mm medial laxity. Femoral and tibial components were cemented in all procedures.

Radiographic Measurement

We took pre- and postoperative AP full-length weightbearing radiographs (hip-knee-ankle). Valgus stress radiography was also preoperatively performed in the supine position to assess the possible correction capability of each knee. A senior orthopaedic surgeon (HO) manually applied the maximum valgus force with the patient feeling no pain to the affected knee, maintaining full knee extension (Fig. 1A). To standardize the rotation, the patella was aligned with the direction of the x-ray beam. The x-ray beam was centered at the distal pole of the patella, aligning the image parallel to the tibial joint line in the frontal plane. The %MA was calculated by measuring the distance from the medial edge of the proximal tibia to the point where the mechanical axis intersects the proximal tibia, then dividing that measurement by the entire width of the proximal tibia (Fig. 1B). We calculated Δ%MA as follows: %MA on postoperative weightbearing radiograph - %MA on preoperative valgus stress radiograph. HKAA was defined as the angle between the femoral mechanical axis (center of the hip to the center of the knee) and the tibial mechanical axis (center of the knee to the center of the ankle). Varus was defined as an hip-knee-ankle angle less than 180° (Fig. 1C). Using Spearman’s correlation coefficient, we found the relationships between the values observed on the preoperative valgus stress radiograph and those on postoperative weightbearing radiographs correlated well (%MA; r = 0.706, hip-knee-ankle angle; r = 0.674) (Fig. 2A-B).

Fig. 1 A-C.

Fig. 1 A-C

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These radiographs show measurement of the % mechanical axis (%MA) and hip-knee-ankle angle (HKAA). (A) Preoperative full-length valgus stress radiography was performed in the supine position. The arrow indicates the mechanical axis. (B) The %MA was calculated as follows: b/a × 100. (C) The HKAA was defined as the angle between the femoral mechanical axis and the tibial mechanical axis.

Fig. 2 A-B.

Fig. 2 A-B

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These graphs show the correlation between valgus stress radiography and postoperative full-length weightbearing radiography. (A) A strong correlation was found between the % mechanical axis (%MA) values on valgus stress radiography and postoperative full-length weightbearing radiography (r = 0.706; p < 0.001). (B) A strong correlation was also observed between the hip-knee-ankle angle (HKAA) values on valgus stress radiography and postoperative full-length weightbearing radiography (r = 0.674; p < 0.001).

Evaluation of Medial Meniscus and Osteophytes with MRI

Two observers (KI, ES), who were blinded to patient background data, quantified these measurements using Synapse ver.2.96 (Fuji Film., Ltd., Minato, Tokyo). The patients’ knees were positioned at 30° of flexion on a mat. MRI scanning was performed using a knee coil and magnetic resonance unit (1.5 T; vantage Titan, Toshiba, Kawasaki, Japan) preoperatively. The distance from the outermost edge of the medial meniscus to a line connecting the femoral and tibial cortices was defined as the medial meniscal extrusion distance [11] on proton density-weighted imaging (repetition time 3800 ms, echo time 70 ms, field of view 16 cm, 256 × 432 matrix, and slice thickness of 4 mm with a gap between slices of 0.8 mm) (Fig. 3A). The interobserver reproducibility was 0.98 (95% CI 0.95 to 0.99) for the MME distance. The preoperative osteophyte areas of the medial femur and tibia were also measured with T1-weighted imaging in the coronal plane (repetition time 550 ms, echo time 12 ms, field of view 16 cm, 256 × 400 matrix, and slice thickness of 4 mm with a gap between slices of 0.8 mm). The osteophyte area was measured by tracing the surface at the slice where the largest osteophyte was observed (Fig. 3B). The interobserver reproducibility was 0.93 (95% CI 0.86 to 0.97) for femoral osteophytes and 0.91 (95% CI 0.63 to 0.97) for tibial osteophytes. To evaluate the influence of medial meniscal extrusion and the size of osteophytes on Δ%MA, the association between the Δ%MA and medial meniscal extrusion distance, femoral osteophyte size, and tibial osteophyte size were evaluated using Spearman’s rank correlation coefficient. The values of Δ%MA correlated with medial meniscal extrusion distance (r = 0.345; p = 0.001) (Fig. 4A), femoral osteophyte area (r = 0.314; p = 0.001) (Fig. 4B), and tibial osteophyte area (r = 0.202; p = 0.041) (Fig. 4C).

Fig. 3 A-B.

Fig. 3 A-B

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These images show measurement of the medial meniscal extrusion distance and osteophyte area on MRI. (A) The medial meniscal extrusion distance was measured as the distance from the outermost edge of the medial meniscus to a line connecting the femoral and tibial cortices. (B) The osteophyte areas of the medial femur and tibia were measured in the coronal plane.

Fig. 4 A-C.

Fig. 4 A-C

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These graphs show the correlations between the Δ% mechanical axis (%MA) and magnetic resonance imaging values. Δ%MA was defined as follows: %MA on postoperative full-length weightbearing radiography - %MA on a preoperative valgus stress radiography. The subpanels show the correlation (A) between Δ%MA and the medial meniscal extrusion distance (r = 0.345; p < 0.001), (B) between Δ%MA and the femoral osteophyte area (r = 0.314; p = 0.001), and (C) between Δ%MA and the tibial osteophyte area (r = 0.202; p = 0.041).

Statistical Analysis

All values in this study were reported as mean and SD. We estimated the correlation between the %MA on postoperative full-length weight-bearing radiographs using a linear regression analysis, with the %MA on preoperative valgus stress radiograph, medial meniscal extrusion distance, femoral osteophyte size, and tibial osteophyte size as dependent variables. The correlation model was established based on only the selected parameters using the stepwise method, with variables entered and excluded for p > 0.05. To validate this model, we estimated a multiple correlation coefficient. To estimate the predictive cutoff values for postoperative overcorrection (%MA > 50%) on postoperative full-length weightbearing radiographs, we performed a receiver operating characteristic (ROC) curve analysis with these parameters. We plotted the false-positive fraction against the one-true-positive fraction, and the cutoff point was defined as the point of the maximum slope that was judged by the area under the curve. The cutoff value and its odds ratio were estimated. All analyses were performed using a commercial statistical package (SPSS® version 22.0, SPSS Inc, Chicago, IL, USA). P values less than 0.05 were considered significant.

Results

In general, %MA on the valgus stress radiograph and medial meniscal extrusion distance correlated with postoperative lower limb alignment after UKA. The estimated %MA on the postoperative weightbearing radiograph was equal to -0.27 + 0.86% (%MA on valgus stress radiograph) + 1.14 mm (medial meniscal extrusion distance) (Table 2). The coefficient of determination (r2 [adjusted]) in this regression model was 0.53. The mean (range) %MA on the valgus stress radiograph was 28 ± 10% (0 to 53), and the correlation coefficient between the %MA on the valgus stress radiograph and %MA on the postoperative weightbearing radiograph was 0.72 (p < 0.001). The mean (range) medial meniscal extrusion distance was 8 ± 2 mm (2 to 12), and the correlation coefficient between mean medial meniscal extrusion distance and %MA on the postoperative weightbearing radiograph was 0.20 (p = 0.003).

Table 2.

Multivariate analyses of the factors associated with %MA on postoperative full-length weightbearing radiographs

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Using a cutoff point of 36%, the %MA on the valgus stress radiograph was associated with overcorrection after UKA (area under the curve 0.89; odds ratio [OR]14 [95% CI 0.75 to 0.95]; p < 0.001, sensitivity 77.8%, specificity 80.9%) (Fig. 5). The other parameters (medial meniscus extrusion distance, tibial osteophytes area and femoral osteophyte area) could not detect the predictive cutoff values for postoperative overcorrection (Fig. 5).

Fig. 5.

Fig. 5

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This image shows receiver operating characteristic curves for the % mechanical axis (%MA) on valgus stress radiography and overcorrection after unicompartmental knee arthroplasty; TPF = true-positive fraction; FPF = false-positive fraction.

Discussion

Postoperative lower limb alignment is important for successful medial UKA [13, 23, 29]. Osteoarthritis progression in the lateral compartment may be caused by overcorrected lower-limb alignment [4]. On the other hand, severe under-correction can cause insert wear and loosening on the tibial side [3]. To predict the postoperative lower limb alignment, the use of preoperative valgus stress radiographs remains controversial [15, 29, 31]. We asked whether routine preoperative valgus stress radiographs and MRI findings could be associated with postoperative lower limb alignment. We found that preoperative valgus stress radiographs and MRI findings were both able to predict the outcome, as they showed good correlations to the postoperative lower alignment. Furthermore, we found a good predictive cut off value for overcorrected postoperative lower limb alignment.

This study has several limitations. First, the valgus stress radiographs were performed manually, rather than using a tensor device. This may have resulted in some variability in our measurements. However, this approach in common practice, and the surgeon who performed the test was experienced, and he tried to perform it as consistently as possible, accounting for each patient’s comfort [30]. Second, we measured the largest osteophyte area with MRI and removed osteophytes as much as possible. However, because we do not believe it is a good idea to release any part of the medial collateral ligament during UKA, it is possible we may not have removed all osteophytes in all patients; again, we believe this is a real-world limitation that reflects common practice. Third, we did not evaluate any postoperative knee scores. Patient-based outcome scores, such as knee injury and osteoarthritis outcome scores can evaluate and detect changes in knee pain and quality of life in patients undergoing knee surgery [26]. In future studies, patient-reported subjective outcomes should be investigated. Fourth, previous studies reported that medial meniscus tears at the posterior segment resulted in displacement of the meniscus and osteoarthritis progression [6, 7]. In our study, 29% (31 of 105) of knees had medial meniscus tears at the posterior segment that were evaluated by MRI. However, we did not assess meniscus tears by arthroscopy before surgery. A medial meniscus tear at the posterior segment might also influence the postoperative lower-limb alignment.

In the present study, the %MA on the valgus stress radiography and medial meniscal extrusion distance were correlated with postoperative lower limb alignment after UKA. Valgus stress radiographs have been advocated as a means by which to predict lower-limb alignment after UKA. Zhang et al. [34] evaluated 79 knees using the valgus stress radiograph and reported that changes in the preoperative and postoperative varus angles strongly correlated with the femorotibial facet angle value and its change on valgus stress radiographs. Another study reported that the postoperative hip-knee-ankle angle was successfully restored to within 3° of the unaffected lower limb’s alignment in 87% of patients [21]. However, Kreitz et al. [15] concluded that the preoperative valgus stress radiograph overstated the value because the full extent to which a varus deformity could be corrected could not be determined until after osteophytes were removed.

Whether removal of osteophytes and the meniscus would affect lower-limb alignment in UKA was unclear. In TKA studies, although medial soft tissue release has historically been considered an important step in TKA for the varus knee, several authors have recently advocated that no medial soft tissue release be performed, claiming this technique helps to achieve proper ligament balance and lower limb alignment [8, 19, 32]. Proper bone resection and removal of the menisci and osteophytes could achieve neutral alignment. Yagishita et al. [33] measured the joint gap at each surgical step in TKA. They reported that resection of medial osteophytes resulted in a 1.9 ± 2.5 mm increase in the gap medially and 0.7 ± 1.2 mm laterally in extension, and a 1.7 ± 1.6 mm increase medially and 1.3 ± 2.0 mm increase laterally in flexion. Although medial osteophyte removal widened the medial gap in TKA, the osteophyte size did not affect the postoperative lower-limb alignment in the present study. This might be because patients undergoing UKA have a relatively early stage of knee osteoarthritis compared with TKA patients.

Medial meniscal extrusion detected by ultrasonography is useful to detect the progression of radiographic knee osteoarthritis [5]. Based on the findings obtained with conventional MRI, the edge of the medial meniscus was considered more medially located than that of medial tibial osteophytes [12]. We thought that removing the meniscus and osteophytes, excluding the medial collateral ligament, might widen the medial gap. According to our regression analysis, although the valgus stress radiograph alone was correlated with postoperative lower-limb alignment (r2 = 0.49), alignment had a higher correlation if the medial meniscal extrusion distance was also considered (r2 = 0.53).

We found that a cutoff of 36% for %MA on the valgus stress radiograph made before UKA was associated with overcorrection after UKA (area under the curve 0.89); The sensitivity/specificity of this cutoff point was 77.8/ 80.9%. Ahn et al. [1] reported the risk factor of postoperative malalignment (3° more or less than neutral alignment in hip-knee-ankle) in medial UKA using multivariate logistic regression. They identified the preoperative distal femoral varus angle and femorotibial angle on valgus stress radiography as risk factors. As the distal femorotibial angle on valgus stress radiography increased by 1°, the undercorrection risk was about 2.7 times higher. Our ROC analysis showed good area under the curve, sensitivity, and specificity. Although the role of valgus stress radiography is controversial, these results support that valgus stress radiography is useful for the preoperative assessment of patients who are potential candidates for medial UKA. Based on our results, the surgeon should be careful to avoid overcorrection in patients with preoperative %MA on valgus stress radiograph greater than 36%. Patients with %MA on valgus stress radiography approaching greater than 36% are at risk of overcorrection if the correction magnitude is higher than that predicted by valgus stress radiographs compared with patients with a %MA on valgus stress radiography less than 36%. In addition, among patients with a %MA on the valgus stress radiography more than 36%, those with higher magnitudes of medial meniscal extrusion area may be at highest risk.

In conclusion, the ability to overcorrect a varus knee with a valgus stress radiograph before UKA and more extrusion of the medial meniscus on preoperative MRI were associated with an increased likelihood of overcorrected alignment after UKA. In patients whose %MA is corrected more than 36% by valgus stress, we suggest the surgeon take particular care to assess the possible complications after medial UKA. In future studies, we plan to conduct long-term evaluation of patients who are candidates for medial UKA detected by our cutoff point (%MA > 36%) with regard to progression of lateral knee osteoarthritis to decide whether or not to avoid UKA. We must also account for patient factors such as age in future studies.

Footnotes

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.

Each author certifies that neither he or she, nor any member of his or her immediate family, has commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.) that might pose a conflict of interest in connection with the submitted article.

Clinical Orthopaedics and Related Research® neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDA approval status, of any drug or device before clinical use.

Each author certifies that his or her institution approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.

This work was performed at Akita Hospital, Noshiro, Japan.

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