Can a total knee arthroplasty be both rotationally unconstrained and anteroposteriorly stabilised? A pulsed fluoroscopic investigation

Abstract

Objectives: Throughout the 20th Century, it has been postulated that the knee moves on the basis of a four-bar link mechanism composed of the cruciate ligaments, the femur and the tibia. As a consequence, the femur has been thought to roll back with flexion, and total knee arthroplasty (TKA) prostheses have been designed on this basis. Recent work, however, has proposed that at a position of between 0° and 120° the medial femoral condyle does not move anteroposteriorly whereas the lateral femoral condyle tends, but is not obliged, to roll back - a combination of movements which equates to tibial internal/ femoral external rotation with flexion. The aim of this paper was to assess if the articular geometry of the GMK Sphere TKA could recreate the natural knee movements in situ/in vivo.

Methods: The pattern of knee movement was studied in 15 patients (six male: nine female; one male with bilateral TKAs) with 16 GMK Sphere implants, at a mean age of 66 years (53 to 76) with a mean BMI of 30 kg/m(2) (20 to 35). The motions of all 16 knees were observed using pulsed fluoroscopy during a number of weight-bearing and non-weight-bearing static and dynamic activities.

Results: During maximally flexed kneeling and lunging activities, the mean tibial internal rotation was 8° (standard deviation (sd) 6). At a mean 112° flexion (sd 16) during lunging, the medial and lateral condyles were a mean of 2 mm (sd 3) and 8 mm (sd 4) posterior to a transverse line passing through the centre of the medial tibial concavity. With a mean flexion of 117° (sd 14) during kneeling, the medial and lateral condyles were a mean of 1 mm (sd 4) anterior and 6 mm (sd 4) posterior to the same line. During dynamic stair and pivoting activities, there was a mean anteroposterior translation of 0 mm to 2 mm of the medial femoral condyle. Backward lateral condylar translation occurred and was linearly related to tibial rotation.

Conclusion: The GMK Sphere TKA in our study group shows movements similar in pattern, although reduced in magnitude, to those in recent reports relating to normal knees during several activities. Specifically, little or no translation of the medial femoral condyle was observed during flexion, but there was posterior roll-back of the lateral femoral condyle, equating to tibiofemoral rotation. We conclude that the GMK Sphere is anteroposteriorly stable medially and permits rotation about the medial compartment.Cite this article: Professor G. Scott. Can a total knee arthroplasty be both rotationally unconstrained and anteroposteriorly stabilised?: A pulsed fluoroscopic investigation. Bone Joint Res 2016;5:80-86. DOI: 10.1302/2046-3758.53.2000621.

Keywords: TKA; knee fluoroscopy; knee kinematics; total knee arthroplasty.

The GMK Sphere prosthesis incorporates a spherical medial tibiofemoral articulation (left) to provide anteroposterior stability and a flat lateral tibial surface to permit longitudinal rotation (right).

Fig. 2

The femoral condyles show an externally rotated orientation in maximum flexion during lunging (light orange) and kneeling (dark orange) activities. Mean condylar position is indicated by the light and dark orange lines, the colour-filled rectangles show the one standard deviation, and the black bars indicate the extreme range for all knees in the cohort. All translations were normalised to a medium-sized tibial baseplate and referenced to a medial/lateral line (grey) passing through the deepest point of the tibial insert concavity and parallel to the posterior baseplate notch. Normalised medial condylar position was more posterior during lunging than during kneeling activities (t-test, p = 0.04). (AP, anteroposterior).

Fig. 3

The femoral condyles rotate about a fixed medial centre during the step-up/down activity. Generally, knees rotated from 1° to 6° tibial internal rotation as the femur flexed from 0° to 75° flexion. The figure shows the mean position and rotation of the femoral condyles with respect to a medium-size tibial baseplate during the step-up/down activity. (AP, anteroposterior)

Knee kinematics during the step-up activity were characterised by little medial condylar translation with highly individual lateral condylar translation and tibial rotation. (a) Tibial internal rotation generally increased with flexion but this was highly individual. (b) There was very little medial condylar translation. (c) There was a mean 5 mm posterior translation of the lateral condyle but again it was highly individual. The thick orange line represents the mean for the group, and the shaded orange area represents one standard deviation. The thin black lines represent the individual data for each knee. (AP, anteroposterior)

Fig. 5

Femoral translations during pivoting activity show predominantly lateral condylar movement about a stationary medial condyle. This diagram superimposes mean locations of the lowest condylar points onto a medium-size baseplate (AP, anteroposterior).

During pivoting activity, there was little medial condylar translation (a) and lateral condylar translation (b) was linearly related to tibial rotation (R² = 0.98). The thick orange line shows the group mean, the shaded orange area represents one standard deviation, and the thin grey lines are data for individual knees. Positive values in each graph represent an anterior translation with negative values showing a posterior position (AP, anteroposterior).