2015 ISAKOS Biennial Congress ePoster #1917

Patella Stabilization Creates a More Congruous Joint Throughout Knee Movement

Damian A. Clark, FRCS, Calgary, AB UNITED KINGDOM
Danielle Simpson, BSc, Bristol UNITED KINGDOM
Andrew J Hughes, FRCS, New York UNITED STATES
Robert Colborne, PhD, Palmerston North UNITED KINGDOM
Jonathan Eldridge, FRCS, Bristol UNITED KINGDOM
Bristol Royal Infirmary, Bristol, UNITED KINGDOM

FDA Status Not Applicable

Summary: Patella stabilisation surgery increases joint congruity throughout knee flexion and may help prevent overloading of cartilage.

Abstract:

Background

Patella dislocations represent a total loss of joint congruity with the femoral trochlea. What relationship there is between a history of dislocation and intervening joint congruity is unknown.

Aim

The study aims to ascertain if:
1. Normal patellofemoral joints remain congruous through flexion.
2. Unstable patellofemoral joint do not remain congruous through flexion.
3. Stabilisation surgery for patella instability increases the congruity of the patellofemoral joint.

PATIENTS
Twenty patients who had experienced multiple patella dislocations and who were awaiting stabilisation surgery were recruited. The primary aetiology was identified as patella alta in eight cases, traumatic medial patellofemoral ligament (MPFL) rupture in six cases and as trochlea dysplasia in six cases. Six volunteers with normal knees were also assessed.

Methods

The patients underwent assessment of joint congruity throughout flexion by dynamic magnetic resonance imaging. Scans were performed in 0, 20 and 40 degrees of flexion, the quadriceps were activated during the scan by asking the subjects to push against a deflating ball. The congruity of the joint was assessed by a previously validated method; direct measurement of the width of cartilage in contact between patella and femur in each axial slice was made and taken to represent the width throughout that 5mm slice. Contact in each axial image was summed to produce a total joint contact area. Contact area was plotted by patient group at 0, 20 and 40 degrees of knee flexion. The assessment was performed before surgery and then a year later.

Results

The six control subjects demonstrated a mean contact area of 4.3/5.3/5.5 cm2 at 0,20 and 40 degrees respectively.
For those patients with patella alta the mean contact area was 0.9/2.7/3.1 cm2 preoperatively and 3.1/4.2/5.1 cm2 postoperatively.
For those patient with traumatic MPFL rupture the mean contact area was 1.7/2.1/4.1 cm2 preoperatively and 2.8/4.6/5.3 cm2 postoperatively.
For those patients with trochlea dysplasia the mean contact area was 1.9/2.8/2.9 cm2 preoperatively and 4.9/6.2/7.2 cm2 postoperatively.

As the knee flexed the mean contact area increased under all conditions. Friedman’s ANOVA rank test evaluated the significance of differences in knee position. Patients with patella alta demonstrated a significant paucity of congruity in extension X2(2)= 12.5 p=0.002, as did patients with a traumatic MPFL rupture X2(2)= 12.5, p=0.002.
In the control group the congruity was well maintained in all positions, joint position having no significant effect on congruity X2(2)=5.3, p=0.07. In patients with trochlea dysplasia (2)=2, p=0.37 the contact was universally poor and knee, flexion did not significantly influence congruity.
Contact area increased significantly between the preoperative and postoperative cases (Wilcoxon, P<0.05) in each patient group. Interobserver correlation was significant (Pearson’s k=0.909 significant at <0.001 significance level).

Conclusion

The normal knee maintains patellofemoral congruence throughout flexion, this is reduced in cases of patella instability and improved by stabilisation surgery. How much a reduced articulating area contributes to cartilage degeneration and stiffness is unknown. Correcting instability increases cartilage contact and not only avoids harmful dislocations but may also normalise the force applied to that cartilage.