The natural history of anteroposterior laxity and its role in knee osteoarthritis progression

Objective. To test the hypotheses that 1) osteoarthritic (OA) knees at more advanced stages have less anteroposterior (AP) laxity compared with OA knees at milder stages, 2) AP laxity decreases over time, and 3) the absence of a decrease in AP laxity is associated with greater progression of medial tibiofemoral OA. Methods. The study group comprised 230 patients with knee OA (75% women, mean age 64 years, mean body mass index [BMI] 30 kg/m²). At baseline and 18 months, AP laxity was measured (in millimeters of tibial translation, under AP shear loading), and semiflexed AP knee radiographs (with knee position confirmed by fluoroscopy) were obtained. Osteophytes were graded for each compartment, using a scale of 0-4. Disease progression was measured as the amount of medial joint space loss between baseline and followup, using linear regression with generalized estimating equations. Results. At baseline, measurements of AP laxity were lower in knees with a Kellgren/Lawrence (K/L) score of 4 (mean ± SD 5.0 ± 2.1 mm) than in those with a K/L score of 0-1 (mean ± SD 7.1 ± 2.6 mm). There was a weak negative correlation between osteophyte grade and AP laxity. In knees with a K/L score of 0-2, AP laxity was slightly lower at 18 months than at baseline. AP laxity at baseline was not a predictor of progression of OA. Knees without a decrease in AP laxity had a greater loss of medial joint space (0.22 mm greater, after adjusting for age, sex, and BMI) than did knees in which laxity decreased. Conclusion. AP laxity at baseline is not predictive of progression of OA. Although knees with a K/L score of 4 had less AP laxity than those with a K/L score of 0-1, most of this difference was attributable to the significant difference in AP laxity between knees with a K/L score of 0-1 and knees with a K/L score of 2 (i.e., definite osteophytes). Knees in which AP laxity decreased had less medial joint space loss than did knees without a decrease in AP laxity. The knee joint may successfully compensate for AP laxity; the absence of such compensation may have a deleterious effect.