Summary

Varus-valgus constrained implants were able to satisfactorily address one third of the evaluated knees with complete MCL insufficiency, with all dislocations in the remaining two thirds presenting only under maximum valgus manual stress at 90o flexion, a scenario which is unlikely to happen during activities of daily living.

Abstract

Objective

The main purpose of the present study was to evaluate if a varus-valgus-constrained TKA can provide sufficient stability in the presence of a complete MCL insufficiency. Secondarily, we aimed to evaluate which are the main parameters related to stability in the setting of a varus-valgus constrained TKA performed at complete MCL insufficiency.

Materials And Methods

Forty-two experienced knee surgeons performed a TKA and subsequent revision TKA in 42 fresh frozen cadaver knees. After revision TKA, trials were assembled and introduced. The following data was collected: femoral size, tibial size, and polyethylene trial insert size. The polyethylene trial insert was then removed and gap measurements data were collected using two spreaders with femoral and tibial trials positioned. A complete transverse mid-substance MCL lesion was made and valgus stability under maximum manual stress at 30 and 90 degrees was applied to evaluate stabilization. Consecutively thicker constrained inserts were tested until valgus stability was obtained or the knee exhibited 15 degrees or more of flexion due to overstuffing of the extension gap by the insert. T-student tests for quantitative variables and chi-square or Fisher tests for categorical variables were conducted. A power analysis was performed resulting in a minimum sample size of 26 specimens for a power of 80%, considering a fixed type I error at 5%.

Results

Upon maximum valgus manual stress testing of the MCL-deficient revision TKA knee, 31% of constructs (13 specimens) presented sufficient stability without unacceptable flexion contracture. All the dislocations with the thickest polyethylene insert without unacceptable flexion contracture happened at 90 degrees with concurrent extensor mechanism lateral displacement. The most notable finding was the absence of resulting stable constructs from specimens with a previously unbalanced flexion gap. When assessing for differences between stable and unstable final constructs, stable constructs displayed significantly (p = 0.015) smaller polyethylene insert size after MCL lesion. No statistically significant differences were found for femoral component size, tibial component size, femoral-tibial component size ratio or polyethylene insert size before MCL transection.

Conclusion

Varus-valgus constrained implants were able to address one-third of the evaluated knees with complete MCL insufficiency, with all dislocations of the remaining two-thirds presenting only under maximum valgus manual stress at 90o flexion. These findings suggest that a varus-valgus constrained implant can be sufficient to address a complete MCL insufficiency in a considerable number of cases as it is unlikely the patients in the clinical settings will be submitted to similar stress at 90o as the maximum manual valgus test at daily living activities.