[Refixation of osteochondral fragments with resorbable polylactid implants : Long-term clincal and MRI results]

BACKGROUND

Refixation with resorbable implants is a common surgical treatment in patients who suffer an injury with shearing of an osteochondral flake due to trauma of the knee or the upper ankle joint. To date there are no studies which outline long-term outcomes for this procedure. The aim of this study was to evaluate long-term clinical and magnetic resonance imaging (MRI) results after refixation with resorbable polylactide (PLLA) implants.

MATERIAL AND METHODS

In this retrospective study 12 patients with 13 injuries were examined 13.9 years (±1.2 years) after refixation of an osteochondral fragment of the knee (10 patients) and the upper ankle joint (2 patients) with a mean size of 3.33 cm² (±2.33) by resorbable polylactide (PLLA) implants (nails, pins, screws, Bionx, Tampere, Finland). To objectify the clinical results eight established clinical scores (VASS, Tegner, Lysholm, McDermott, KSS, WOMAC, AOFAS, FADI+Sports) were used. Furthermore, the morphological integration of bone and cartilage was assessed by MRI (3 T) using proton-weighted and cartilage-sensitive 3D double-echo steady-state (DESS) sequences. The morphological results were objectified with a modified MRI score according to Henderson et al.

RESULTS

After 13.9 years (±1.2) the patients with an injury of the knee as well as of the upper ankle joint showed good to excellent results (knee: VASS 1.2 (±1.7), Tegner 4.4 (±1.3), Lysholm 85.7 (±12.2), McDermott 90.7 (±8.6), KSS 189 (±14.2), WOMAC (6.16% (±8.45)) (upper ankle joint: VASS 2.5 (±2.5), Tegner 5.5 (±1.5), Lysholm 87 (±13), McDermott 88 (±12); WOMAC (8.54% (±8.54), AOFAS 75.5 (±24.5), FADI+Sports 118 (±18)). In all cases there was evidence of good integration of the osteochondral fragment in MRI. In five patients there was moderate subchondral cyst formation (∅ ≤1 mm); however, mild changes of the cartilage contour were found in all patients. The mean modified Henderson score achieved was 14.4 (±2.0, best 8, worst 32), which corresponds to a good morphological result.

CONCLUSION

Because of good clinical and morphological results shown by MRI, refixation through resorbable implants (PLLA) can be recommended for treatment of traumatic osteochondral flakes.

A model of polymer degradation and erosion for finite element analysis of bioresorbable implants

Finite element analysis is a powerful tool for the design of bioresorbable medical implants made of aliphatic polyesters such as bioresorbable vascular scaffolds. However polymer erosion has been traditionally modelled using empirical rules rather than differential equations. The rule-based models are difficult to implement in a finite element analysis. Consequently, these models have been limited to simple geometries such as plates or spheres. This paper presents a set of differential equations that govern the hydrolytic chain scission and bulk erosion of bioresorbable implants where polymer erosion is modelled using a differential equation instead of empirical rules. These differential equations can be conveniently solved using a commercial finite element package to calculate the molecular weight and mass loss as functions of time for bioresorbable implant made of aliphatic polyesters. A case study of Absorb bioresorbable vascular scaffolds (BVSs) is presented using data obtained from the literature, where 98 Absorb BVSs were implanted in 40 porcine coronary arteries. It is demonstrated that the finite element model can fit the data of both molecular weight and mass loss as functions of time to an accuracy of approximately 5%. The finite element model and the back-calculated model parameters can be used to design future implants that degrade in a controlled pattern with required mechanical performance.