Analysis of Medial Flexion Gap After Medial Release for Varus Deformity by Navigation-guided TKA

Abstract

The goal of this study was to analyze medial flexion gaps after medial release for varus deformity by navigation-guided total knee arthroplasty (TKA). In each patient, a preoperative standing anteroposterior (AP) radiograph of the lower extremity and an AP valgus stress radiograph of the knee were used to measure preoperative mechanical axis angle and valgus stress angle, respectively. The correlation between preoperative varus deformities and medial flexion gap increases as measured by navigation was examined. Patients were assigned to 2 groups: group A (25 knees), in which the difference between the lateral flexion gap (LFG) and the medial flexion gap (MFG) (LFG-MFG) was <1 mm; and group B (73 knees), with an LFG-MFG of >1 mm.

Mean preoperative mechanical axis angles in groups A and B were 13.21°±5.01° varus (range, 3.7°-23.6°) and 10.05°±3.70° varus (range, 1.9°-23.7°), respectively. Mean preoperative valgus stress angles in groups A and B were 1.72°±0.89° valgus (range, 0.1°-4.0°) and 4.84°±2.61° valgus (range, 0.1°-11.7°), respectively. A significant difference was observed between the groups in terms of mechanical axis angle (P=.002) and valgus stress angle (P<.001). Furthermore, valgus stress angle was found to be more strongly correlated with medial flexion gap increase than mechanical axis angle. The cutoff values of mechanical axis angle and valgus stress angle in group A were 13.4° and 2.45°, respectively.

This study shows that preoperative valgus stress angle measurements can be used to predict the extent of medial release for varus deformity.

The achievements of proper soft tissue balance and limb alignment are recognized as the most important considerations of successful total knee arthroplasty (TKA). Several authors have described surgical procedures that achieve proper soft tissue balance.^1,2 In varus-deformed knees, ligaments and soft tissues on the medial side undergo contracture, and thus must be released to achieve neutral limb alignment.^3,4 In practice, it is difficult to achieve neutral alignment when performing TKA in these knees. In particular, after performing medial soft tissue release to achieve neutral limb alignment for severely varus-deformed knees, it is difficult to balance the flexion gap due to abrupt increase in the medial flexion gap (Figure 1), which can cause many adverse effects after TKA. Occasionally, a thicker polyethylene insert or a more constrained prosthesis should be used.⁵ Furthermore, maltracking of the patella has been reported because of derotation of the femoral component.⁶

Figure 1: Standing AP radiographs (left) and screenshots (right) of a navigation system-measured flexion gap. The lateral flexion gap was wider than the medial flexion gap after medial release, and achieved acceptable alignment for mild varus deformity (A). The medial flexion gap was wider than the lateral flexion gap after medial release, and achieved acceptable alignment for severe varus deformity (B).

We hypothesized that preoperative varus deformity severity affects medial flexion gap increase after medial soft tissue release has been performed to achieve neutral limb alignment. The goal of this study was to analyze quantitatively the correlation between preoperative severities of varus deformity and medial flexion gap increases after medial soft tissue release to achieve acceptable alignment using a navigation system, and to investigate whether preoperative varus deformity severity predicts an increase of medial flexion gap after TKA.

Material and Methods

We retrospectively reviewed 135 knees in 85 patients who underwent navigation-guided primary TKA from September 2006 to April 2009 at our institution. Of these 135 knees, 98 knees (65 patients) with preoperative varus mechanical axis alignment were checked using preoperative radiographs and had acceptable intraoperative mechanical axis alignment (within ±3° from neutral alignment) as determined using a navigation system. These 98 knees (8 men and 90 women) constituted the study cohort. Mean patient age was 68.2 years (range, 56-81 years). All patients had osteoarthritis of the knee. Preoperative ranges of knee motion were retrospectively reviewed by chart review. Mean preoperative knee flexion contracture was 8.11°±6.11° (range, 3°-25°), and the mean preoperative knee maximum flexion angle was 132.09°±13.92° (range, 95°-145°).

All TKAs were performed by a single surgeon (Y.W.M.) using a navigation system (OrthoPilot; B. Braun Aesculap, Tuttlingen, Germany). This navigation system is an image-free system that uses kinematic analysis of the hip, ankle, and knee joints, and anatomic registration of the knee joint to construct a working model of the knee. E.motion UC (Ultra-Congruency; B.Braun Aesculap) implants were used throughout.

Preoperative varus deformity was assessed radiographically. To assess varus deformity, a standing anteroposterior (AP) radiograph of the whole lower extremity and an AP valgus stress radiograph of the knee were taken while a valgus stress of 15 lbs (6.8 kg) was applied to the knee in extension using a Telos SE arthrometer (Fa Telos; Medizinisch-Technische, Greisheim, Germany) (Figure 2A).⁷ Midpoints of the distal femoral shaft, which are 10 and 15 cm from the knee joint line, respectively, and the midpoints of the proximal tibial shaft, which are 7.5 and 12.5 cm from the knee joint line, respectively, were also assigned and connected on each AP valgus stress radiograph.⁴ The angle between these lines was measured using a PACS system (Centricity; General Electric, Chicago, Illinois) and was defined as the valgus stress angle (Figure 2B). In each case, the knee mechanical axis was assessed preoperatively using the hip–knee–ankle angle determined using a standing AP radiograph of whole lower extremity.⁸ The hip center, femoral notch center, and ankle center were assigned and connected, and the angle between these lines, defined as the mechanical axis angle of the knee, was also measured using a PACS system (Figure 3). All measurements were performed by 1 investigator (J.G.K.), who was unaware of flexion and extension gap results. Mean preoperative mechanical axis angle was 10.86°±4.28° varus (range, 1.9°-23.7° varus) and mean preoperative valgus stress angle was 4.05°±2.67° valgus (range, 0.1°-11.7°).

Figure 2: Valgus stress radiograph of the knee obtained using a Telos device (A). Measurement of the preoperative valgus stress angle on AP valgus stress radiograph of the knee using the PACS system (B). Figure 3: Measurement of the preoperative mechanical axis angle of the lower extremity on the standing AP radiograph of the lower extremity using the PACS system.

Surgical Technique

A standard medial parapatellar arthrotomy approach was used after a median skin incision was made. The patella was subluxated laterally and the tibia was subluxated anteriorly. The anterior cruciate ligament and posterior cruciate ligament (PCL) were sacrificed, and the medial meniscus and osteophytes were removed. Primary minimal medial release was performed before registration and bone cutting to correct varus deformity in accord with preoperative valgus stress angle (when preoperative valgus stress angle was in an acceptable range [>7°], medial release was not performed at this stage). A screw (1 pin) was then fixed to the medial aspect of the distal third of the femur and to the proximal third of the tibia, respectively, and femoral and tibial trackers were mounted on these pins. The positions of the selected were registered. After marking all reference points, the navigation system was used to check for acceptable limb alignment, which ranged from a mechanical axis alignment of 3° of valgus to 3° of varus. Secondary medial soft tissue release and posterior osteophyte removal were performed at this stage, if required.

Modified medial soft tissue release was performed as described by Mullaji et al.⁹ Initially, the PCL was removed from its femoral and tibial attachment sites. The medial soft tissue and posteromedial capsule were then released from the edge of the tibial joint surface, which included the release of the attachment of the deep medial collateral ligament, and medial osteophytes were removed. If an acceptable alignment was not obtained, further subperiosteal release of the superficial medial collateral ligament was performed using a periosteal elevator, or partial release of the tibial insertion of the semi-membranous was performed until an acceptable range of coronal alignment was achieved, but the pes anserinus was not released at the tibia. Proximal tibial cutting was performed initially in a plane perpendicular to the mechanical axis of the tibia. A tensor with a slide ruler, which allowed the medial and lateral compartments to be separately adjusted, was then inserted into the space between the femur and the osteotomized tibia, and distraction force was applied using a laminar spreader by the surgeon using maximum right hand grip (40 kg) in extension and at 90° of flexion. Medial and lateral gaps during extension and at 90° of flexion were then recorded. Femoral planning was performed using the OrthoPilot navigation system by simulating femoral component sizing, rotation, and the amount of femoral bone cutting required for a balanced gap. All data were recorded in the navigation system.

Gaps in extension and at 90° of flexion and femoral component rotations recorded in the navigation system, and preoperative mechanical axis angle and valgus stress angle values were recorded in an Excel worksheet (Microsoft, Redmond, Washington). The lateral gap is consistently larger than the medial gap in severely varus-deformed knees, but because the medial flexion gap increases abruptly after subperiosteal release of the superficial medial collateral ligament, the difference between the lateral flexion gap (LFG) and the medial flexion gap (MFG) (LFG-MFG) decreases. Thus, we assigned patients to 1 of 2 groups: group A (25 knees), with an LFG-MFG of <1 mm; and group B (73 knees), with an LFG-MFG of >1 mm. All statistical analyses were performed using SAS version 9.1.3 (SAS institute Inc, Cary, North Carolina) and PASW 17.0 (SPSS Inc, Chicago, Illinois). All analyses were set at the 95% confidence interval for statistical significance.

Results

Patient demographic data are summarized in the Table. Groups A and B were similar in terms of age (t test, P=.12), sex (Fisher’s exact test, P=.67), preoperative flexion contracture (Wilcoxon rank-sum test, P=.78), and maximum flexion (Wilcoxon rank-sum test, P=.55).

Mean preoperative mechanical axis angle values in groups A and B were 13.21°±5.01° varus (range, 3.7°-23.6°) and 10.05°±3.70° varus (range, 1.9°-23.7°), respectively. The t test was used to compare the 2 groups after log transforming data due to deviations from normality, and mean preoperative mechanical axis angle values were found to be significantly different in the 2 groups (P=.002). Mean preoperative valgus stress angle values were 1.72°±0.89° valgus (range, 0.1°-4.0°) and 4.84°±2.61° valgus (range, 0.1°-11.7°) for groups A and B, respectively, and these values were significantly different by the t test (P<.001). Mean femoral component rotations were 1.04°±0.93° of external rotation (range, 0.0°-4.0°) and 4.05°±1.14° of external rotation (range, 1.0°-6.0°) for groups A and B, respectively, and this difference was significant by Wilcoxon rank-sum test (P<.001).

The correlation between preoperative mechanical axis angle and preoperative valgus stress angle values was performed using Spearman’s correlation coefficients. A weak negative correlation was found between preoperative mechanical axis angle and preoperative valgus stress angle (correlation coefficient = –0.30; P=.002). Subsequent analysis using generalized estimating equations showed that preoperative valgus stress angle influenced medial flexion gap increase more than preoperative mechanical axis angle. Spearman’s rank correlation analysis was used to examine the relation between preoperative valgus stress angle and LFG-MFG, and a positive correlation was found between the 2 (correlation coefficient=0.56; P<.001).

Receiver operating characteristic curves were constructed to determine the optimal cutoff values for preoperative valgus stress angle, preoperative mechanical axis angle, and femoral component rotation in group A. Based on this analysis, preoperative valgus stress angle cutoff values of <2.45° and <3.15° of valgus resulted in sensitivities of 84% and 96% and specificities of 83% and 73%, respectively. For preoperative mechanical axis angle, a cutoff value of >13.4° of varus had a sensitivity of 60% and a specificity of 84.9%, and for femoral component rotation, a cutoff value of <2.5° of external rotation had a sensitivity of 96% and a specificity of 93%.

Discussion

This study addresses the effect of medial soft tissue release on medial flexion gap during TKA in varus-deformed knees and shows that preoperative varus deformity measured using preoperative mechanical axis angle and valgus stress angle values influences medial flexion gap increase when medial soft tissue release is performed to achieve neutral alignment.

Many authors have assessed the effects of medial soft tissue release on alignment and mediolateral gap changes in flexion and extension.^1,4,10-13 However, few studies have used a navigation system,^3,11 and little has been reported concerning the effect of preoperative varus deformity on gap changes in flexion and extension.

Several methods have been described to address severe varus deformity: subperiosteal release of the superficial medial collateral ligament,^9,14,15 joint line release of the medial collateral ligament,¹⁴ epicondylar osteotomy,^14,16 and tibial reduction osteotomy.¹⁷ Authors have used subperiosteal release of the superficial medial collateral ligament to perform TKA in severely varus-deformed knees. Engh¹⁴ suggested during varus posturing that the major contracting, deforming force is provided by the superficial medial collateral ligament, and that soft tissue stripping on the medial side for varus deformity has the advantage of providing medial stability after secondary scar formation and periosteal healing. However, it is important to preserve the continuity of the superficial medial collateral ligament, which when over-released, can produce excess medial laxity in extension and flexion,⁹ especially in flexion.^4,10,13

The extent of superficial medial collateral ligament release in varus-deformed knees is controversial among authors. Luring et al¹¹ performed a similar study using navigation, and also found that release of the anteromedial sleeve 6 cm below the joint line resulted in a higher gap increase in flexion than in extension. Matsueda et al¹² performed a cadaver study using knees without deformities and reported that release of the anteromedial sleeve 8 cm from the medial joint line created the most significant increase in coronal angulation on the medial side. Therefore, it may be better not to release superficial medial collateral ligament over these limits. However, it is difficult to prevent extensive release of superficial medial collateral ligament in irreducible varus knees. We predicted the possibility of extensive release of superficial medial collateral ligament preoperatively by using preoperative AP valgus stress radiographs and determined the extent of superficial medial collateral ligament release by checking the alignment and the gap using navigation intraoperatively in irreducible knees. If extensive release of superficial medial collateral ligament for irreducibility was necessary, we performed another method to correct varus deformity.

Several methods have been devised that balance ligament tension on medial and lateral sides in severely varus-deformed knees without over-releasing medial soft tissues. It has been suggested that sacrifice of the PCL and the use of a PCL-substituting prosthesis is necessary for the correction of severe varus deformity.¹⁸ However, this method alone cannot correct severe varus deformity; it can be used in combination with another method. Dixon et al¹⁷ suggested that the removal of medial overhang after downsizing and lateralization of the tibial tray could achieve a balanced, stable TKA without additional medial soft tissue release. Sekiya et al¹⁹ found that preoperative varus deformity is closely correlated with lateral ligamentous laxity, and that lateral ligamentous laxity in varus-deformed knees is reduced at 3 months after TKA. It was suggested that some degree of residual lateral laxity is allowable as long as proper valgus alignment is maintained.¹⁹

In the present study, mean preoperative mechanical axis angle in group A was 13.21°±5.01° of varus (range, 3.7°-23.6°, which was not in the severe varus deformity range, and the range of preoperative mechanical axis angle in group A was also wide. The reason for this result is that although varus deformity is moderate, if not correctable in valgus stress (showing lower valgus stress angle), we had the opportunity to perform superficial medial collateral ligament release during navigation TKA. Verdonk et al⁸ suggested that preoperative varus alignment severity determines the type of release needed, but this is also affected by other variables, such as the reducibility of the deformity. In terms of additional soft tissue release, the reducibility of varus deformity is more important than preoperative varus alignment.

We were not able to check valgus stress angle using standing AP radiographs of the whole lower extremity, and thus we defined valgus stress angle as the angle between the anatomical axes of the distal femur and proximal tibia. This differs from the mechanical axis angle determined by measuring hip–knee–ankle angle using standing AP radiographs of the whole lower extremity, because the anatomical axis of the femur is in approximately 7° of valgus from the mechanical axis of lower limb, and thus valgus stress angle is in approximately 7° of valgus from the mechanical axis angle.²⁰ Although preoperative valgus stress angles in group A were valgus or nearly neutral in anatomical axis angle (0.1°-4.0° of valgus), mechanical axis angles were usually in varus. Knees in group A with preoperative valgus stress angles in the range of 0.1° to 4.0° were not correctable, but some knees in group B with preoperative valgus stress angles in the range of 0.1° to 11.7° were correctable. The reason for this is that other factors, such as large osteophytes of the medial femoral condyle or medial tibial plateau, alter preoperative valgus stress angle in the absence of severe contracture of medial soft tissue by tenting medial soft tissues.

One weakness of the present study concerns the use of a laminar spreader with maximal manual tension during medial and lateral gap measurements, given likely grip power differences. Some authors have proposed measuring the joint gap using a 40-lb (18.7 kg) distraction force at full extension using a tensioning device, because it was found that this gap most closely corresponds to the thickness of the insert required for the procedure.²¹ However, Griffin et al²² suggested that gap measurements using a laminar spreader are reproducible when 1 surgeon performs the tensioning for all cases.

Conclusion

Preoperative varus deformity measured using preoperative mechanical axis angle and valgus stress angle values influence medial flexion gap increase when medial soft tissue release is performed to achieve neutral alignment. Preoperatively determined reducibility of varus deformity based on preoperative valgus stress angle was found to be more predictive of a medial flexion gap increase after medial release than preoperative alignment based on preoperative mechanical axis angle.

References

1. Clayton ML, Thompson TR, Mack RP. Correction of alignment deformities during total knee arthroplasties: staged soft-tissue releases. Clin Orthop Relat Res. 1986; (202):117-124.
2. Insall JN, Binazzi R, Soudry M, Mestriner LA. Total knee arthroplasty. Clin Orthop Relat Res. 1985; (192):13-22.
3. Luring C, Bäthis H, Hüfner T, Grauvogel C, Perlick L, Grifka J. Gap configuration and anteroposterior leg axis after sequential medial ligament release in rotating-platform total knee arthroplasty. Acta Orthop. 2006; 77(1):149-155.
4. Yagishita K, Muneta T, Ikeda H. Step-by-step measurements of soft tissue balancing during total knee arthroplasty for patients with varus knees. J Arthroplasty. 2003; 18(3):313-320.
5. Yasgur DJ, Scuderi GR, Insall JN. Surgical Technique in Total Knee Arthroplasty. New York, NY: Springer; 2002.
6. Akagi M, Matsusue Y, Mata T, et al. Effect of rotational alignment on patellar tracking in total knee arthroplasty. Clin Orthop Relat Res. 1999; (366):155-163.
7. Yagishita K, Muneta T, Yamamoto H, Shinomiya K. The relationship between postoperative ligament balance and preoperative varus deformity in total knee arthroplasty. Bull Hosp Jt Dis. 2001; 60(1):23-28.
8. Verdonk PC, Pernin J, Pinaroli A, Ait Si Selmi T, Neyret P. Soft tissue balancing in varus total knee arthroplasty: an algorithmic approach [published online ahead of print March 17, 2009]. Knee Surg Sports Traumatol Arthrosc. 2009; 17(6):660-666.
9. Mullaji AB, Padmanabhan V, Jindal G. Total knee arthroplasty for profound varus deformity: technique and radiological results in 173 knees with varus of more than 20 degrees. J Arthroplasty. 2005; 20(5):550-561.
10. Krackow KA, Mihalko WM. The effect of medial release on flexion and extension gaps in cadaveric knees: implications for soft-tissue balancing in total knee arthroplasty. Am J Knee Surg. 1999; 12(4):222-228.
11. Luring C, Hüfner T, Perlick L, Bäthis H, Krettek C, Grifka J. The effectiveness of sequential medial soft tissue release on coronal alignment in total knee arthroplasty: using a computer navigation model. J Arthroplasty. 2006; 21(3):428-434.
12. Matsueda M, Gengerke TR, Murphy M, Lew WD, Gustilo RB. Soft tissue release in total knee arthroplasty. Cadaver study using knees without deformities. Clin Orthop Relat Res. 1999; (366):264-273.
13. Mullaji A, Sharma A, Marawar S, Kanna R. Quantification of effect of sequential posteromedial release on flexion and extension gaps: a computer-assisted study in cadaveric knees [published online ahead of print May 14, 2008]. J Arthroplasty. 2009; 24(5):795-805.
14. Engh GA. The difficult knee: severe varus and valgus. Clin Orthop Relat Res. 2003; (416):58-63.
15. Whiteside LA, Saeki K, Mihalko WM. Functional medial ligament balancing in total knee arthroplasty. Clin Orthop Relat Res. 2000; (380):45-57.
16. Engh GA, Ammeen D. Results of total knee arthroplasty with medial epicondylar osteotomy to correct varus deformity. Clin Orthop Relat Res. 1999; (367):141-148.
17. Dixon MC, Parsch D, Brown RR, Scott RD. The correction of severe varus deformity in total knee arthroplasty by tibial component downsizing and resection of uncapped proximal medial bone. J Arthroplasty. 2004; 19(1):19-22.
18. Laskin RS. The Insall Award. Total knee replacement with posterior cruciate ligament retention in patients with a fixed varus deformity. Clin Orthop Relat Res. 1996; (331):29-34.
19. Sekiya H, Takatoku K, Takada H, Sasanuma H, Sugimoto N. Postoperative lateral ligamentous laxity diminishes with time after TKA in the varus knee [published online ahead of print October 22, 2008]. Clin Orthop Relat Res. 2009; 467(6):1582-1586.
20. Vail TP, Lang JE. Surgical techniques and instrumentation in total knee arthroplasty. In: Insall JN, Scott WN, eds. Surgery of the Knee. 4th ed. Philadelphia, PA: Churchill Livingstone; 2006:1455.
21. Tanaka K, Muratsu H, Mizuno K, Kuroda R, Yoshiya S, Kurosaka M. Soft tissue balance measurement in anterior cruciate ligament-resected knee joint: cadaveric study as a model for cruciate-retaining total knee arthroplasty [published online ahead of print March 30, 2007]. J Orthop Sci. 2007; 12(2):149-153.
22. Griffin FM, Insall JN, Scuderi GR. Accuracy of soft tissue balancing in total knee arthroplasty. J Arthroplasty. 2000; 15(8):970-973.

Authors

Drs Moon, Woo, Lim, and Seo are from the Department of Orthopedic Surgery, Samsung Medical Center, Sungkyunkwan University School of Medicine, and Dr Kim is from the Department of Orthopedic Surgery, Korea University College of Medicine, Guro Hospital, Seoul, South Korea.

Drs Moon, Kim, Woo, Lim, and Seo have no relevant financial relationships to disclose.

Correspondence should be addressed to: Jae Gyoon Kim, MD, Department of Orthopedic Surgery, Samsung Medical Center, 50 Ilwon-Dong, Kangnam-Ku, Seoul, 135-710, South Korea (gowest99@naver.com).

doi: 10.3928/01477447-20110317-10