Abstract
Background:
Varus-valgus constrained (VVC) implants are used in cases of severe valgus deformity, attenuated medial collateral ligament and difficulty in balancing the medial and lateral gaps of the knee. The increased constraint has been postulated to lead to more stress at the bone-implant interface and early loosening. The objective of this study was to compare the wear characteristics of the polyethylene liner in VVC prosthesis with the posterior-stabilized (PS) prosthesis and identify the factors leading to more wear in the VVC tibial inserts.
Methods:
This was a retrieval analysis of all VVC liners collected from patients who underwent revision surgery from 1999 to 2011. These patients were matched to another group with posterior-stabilized inserts who underwent revision in the same time period. These two groups of patients were similar in terms of their demographic data and implant dimensions. Inserts were divided into 16 zones and a microscopic analysis of surface damage was carried out. We determined overall damage with a scoring system. Pre-revisions radiographs were reviewed and analyzed for correlation with the wear profile.
Results:
There was significantly more damage in the posts of the VVC group (13.0 ± 5.0, compared to 4.7 ± 1.9 in the PS group) (p < 0.001). There was no difference in the backside wear, or wear in the medial and lateral compartments. Within the VVC group, the total damage score and cold flow damage were significantly higher with excessive joint line changes (≥ 5 mm). The excessive joint line elevation was associated with rotational wear pattern of the post (p = 0.004). The total abrasion and pitting scores were also higher in knees without proper restoration of the limb alignment (> 3° varus or valgus). Femoral component malposition correlated with higher total cold flow, pitting, debris damage scores, wear in medial and lateral compartments, and the anterior portion of the insert. The total damage scores for the posts in the VVC group were significantly higher with anterior tibial slope, and tibial component malposition. The most common cold flow deformation was found in the posterior post, followed by the anterior post. The cold flow damage to the post was significantly higher in excessive joint line changes (anterior and posterior post) and in tibial malposition (medial and lateral post).
Conclusion:
The increased constraint resulted in more post damage. Joint line elevation, femoral and tibial component malposition, and anterior tibial slope resulted in significantly more wear in the VVC inserts.
