Total knee arthroplasty in valgus knees: predictive preoperative parameters influencing a constrained design selection.

INTRODUCTION: In valgus knees, ligament balance might remain a challenge at total knee prostheses implantation; this leads some authors to systematically propose the use of constrained devices (constrained condylar knee or hinge types...). It is possible to adapt the selected level of constraints, by reserving higher constraints to cases where it is not possible to obtain final satisfactory balance: less than 5 degrees of residual frontal laxity in extension in each compartment, and a tibiofemoral gap difference not in excess to 3 mm between flexion and extension.

HYPOTHESIS: It is possible to establish preoperative criteria that can predict a constrained design prosthetic implantation at surgery.

MATERIALS AND METHODS: A consecutive series of 93 total knee prostheses, implanted to treat a valgus deformity of more than 5 degrees was retrospectively analysed. Preoperatively, full weight bearing long axis AP views A-P were performed: hip knee angle (HKA) averaged 195 degrees (186 degrees to 226 degrees), 36 knees had more than 15 degrees of valgus, and 19 others more than 20 degrees of valgus. Laxity was measured by stress radiographies with a Telos system at 100 N. Fifty-two knees had preoperative laxity in the coronal plane of more than 10 degrees. Fourteen knees had more than 5 degrees laxity on the convex (medial) side, 21 knees had more than 10 degrees laxity on the concave (lateral) side. Statistical assessment, using univariate analysis, identified the factors that led, at surgery, to an elevated constraint selection level; these factors of independence were tested by multivariate analysis. Logistical regression permitted the classification of the said factors by their odds ratios (OR).

RESULTS: High-constraints prostheses (CCK type) numbered 26 out of 93 implantations; the other total knee prostheses were regular posterostabilized (PS) prostheses. Statistically, the preoperative factors that led to the choice of a constrained prosthesis were: (1) valgus severity as measured by HKA (PS=193 degrees, CCK=198 degrees), (2) increased posterior tibial slope (PS=4.8 degrees, CCK=6.5 degrees), (3) low patellar height (using Blackburne and Peel index PS=0.89, CCK=0.77), (4) severity of laxity in valgus (PS=2.3 degrees, CCK=4.3 degrees). Among all these factors, the only independent one was laxity in valgus (convex side laxity) (p=0.0008). OR analysis showed a two-fold increased probability of implanting an elevated constraints prosthesis for each one degree increment of laxity in valgus.

DISCUSSION: This study demonstrated that it was not the valgus angle severity but rather the convex medial side laxity that increased the frequency of constrained prostheses implantation. Other factors, as a low patellar height or an elevated posterior tibial slope, when associated, potentiate this possible prosthetic switch (to higher constraints) and should make surgeons aware, in these situations, of encountering difficulties when establishing ligament balance.

LEVEL OF EVIDENCE IV: Therapeutic retrospective study.