Axial deformity secondary to degenerative joint disease of the knee can modify stress forces. Certain studies have reported an inversely proportional relationship between degenerative disease and osteoporosis. The aim of this prospective study was to quantify the horizontal linear distribution of bone density using dual x-ray absorptiometry (DXA) of the proximal tibia as a function of the femoral neck bone density in patients with knee osteoarthritis.

Between September 1996 and March 1998, 90 cases of primary degenerative joint disease of the knee were programmed for total knee arthroplasty. Prior to the procedure, the patients were assessed clinically and radiologically according to the International Knee Society (IKS) criteria. The mechanical femorotibial angle was measured in all patients and the varus angles were recorded. Most of the patients were women (65 p. 100) with a mean age of 70 ± 5 years. Valgus knees were excluded from this series. The mean mechanical femorotibial angle was 172 ± 5°. Fifteen patients had a normal axis (16 p. 100), 32 had a varus measuring 4° to 10° (35 p. 100) and 43 had a varus measuring 10° or more (48 p. 100). The overall varus distance was 6.4 ± 2 cm. All patients had two DXA explorations: femoral neck to determine the bone status according to the WHO criteria (normal, osteopenia, osteoporosis), knee to determine the linear distribution of bone density of the proximal tibia. A 7 mm high band including 7 regions of interest covering the width of the tibia were explored in the area where the tibial cut was to be made. These 7 regions of interest were: R1, R2 under the lateral compartment, R6, R7 under the medial compartment, and R3, R4, R5 on either side of the tibial spines. The level of significance was set at 5 p. 100.

The mean Z score (0.54 ± 1) in the 90 patients showed a symmetrical distribution. These patients were representative of their age range. Their T score was - 1.40 ± 1 (m ± SD) and most had osteopenia (54 p. 100) according to the WHO criteria, although 16 p. 100 had osteoporosis. Mean bone density of the knee was 0.898 ± 0.163 g/cm ³ and was correlated with that of the femoral neck (r = 0.61, p = 0.001). There were significant correlations between the differences in the bone densities of the knee compartments (R6-R2, R7-R1) and the mechanical femorotibial angle [(r = 0.39, p = 0.0001); (r = 0.52, p = 0.001)]. Irrespective of the overall bone density, there was a strong medial compartment overloading, which correlated with the degree of varus deformation.

DXA assessment of bone mineral density of the proximal tibia is a simple, reliable, precise and reproducible method. The distribution of bone density in the degenerative knee depends on the degree of deformation. The average level depends on the subject's general state of mineralization. Osteoporosis does not protect against degeneration of the knee joint since 16 p. 100 of our patients had osteoporosis according to the WHO criteria.

Dans la gonarthrose, les déviations axiales entraînent une modification des contraintes. L'objectif de cette étude prospective était de quantifier la distribution linéaire horizontale de la densité minérale osseuse (D.M.O) par DXA (Dual X-ray Absorptiometry) du tibia proximal dans la gonarthrose en fonction de la D.M.O. du col fémoral.

90 patients gonarthrosiques ayant un genu varum ou normoaxés évalués selon les critères de l'IKS (International Knee Society) avec une mesure des écarts varisants ont été inclus dans cette étude. Tous ont eu deux examens DXA : un examen du col fémoral pour préciser leur état osseux en fonction des critères de l'O.M.S. (normaux, ostéopéniques, ostéoporotiques), un examen du genou afin d'étudier la distribution linéaire de la densité minérale osseuse de l'extrémité supérieure du tibia.

La minéralisation moyenne du genou était de 0,898 ± 0,163 g/cm ² . La différence de densité entre les deux compartiments du genou était d'autant plus élevée que le genu varum et l'écart varisant intrinsèque étaient importants (r = 0,52, p = 0,001, r = 0,39 ; p = 0,001). La D.M.O. du col fémoral était normale pour 28 patients, 48 étaient ostéopéniques et 14 ostéoporotiques. La minéralisation moyenne du genou était corrélée à la mesure effectuée du col fémoral (r = 0,61, p = 0,001).

La mesure de la D.M.O. linéaire horizontale par DXA du tibia proximal est une méthode simple, fiable, précise et reproductible. La répartition de la minéralisation du genou arthrosique dépend de l'importance de la déformation. Cette différence de densité entre les deux compartiments du genou représente la distribution dans la vie quotidienne des contraintes qui s'exercent sur des genoux arthrosiques. Son niveau moyen dépend de l'état de minéralisation du sujet.