Influence of the antero-posterior position of a pin on the risk of a medial hinge fracture during a lateral opening wedge distal femoral varus osteotomy. A biomechanical study.

INTRODUCTION

Lateral opening wedge distal femoral varus osteotomy (LOWDFO) is indicated for early isolated lateral compartment osteoarthritis in young patients with genu valgum due to an isolated distal femoral metaphyseal deformity. A medial hinge fracture is considered a poor prognostic factor for radio-clinical outcomes, hence its prevention is crucial. The protective role of a temporary pin has been demonstrated in the literature. The primary objective of this study was to evaluate the influence of the antero-posterior position of this pin on hinge protection.

HYPOTHESIS: The main hypothesis was that a centrally positioned pin offers the best resistance to breakage compared to anterior and posterior positions.

MATERIALS AND

METHODS

The experimental model was a 3D model of a distal femur, fabricated and printed from the pre-operative CT scan of a patient who underwent a LOWDFO. ABS polymer was chosen for printing these models due to its fracture characteristics similar to bone. Fifteen specimens were printed. Three different cutting guides were also printed, each corresponding to one of the three pin protection positions, the use of these cutting guides ensure the reproducibility of pin positioning. Three pin positions were defined: 25% anterior and 75% posterior junction (anterior group) on a horizontal plane of the distal femur, 50%-50% junction (central group), and 75% anterior and 25% posterior junction (posterior group). The tests were conducted on a single-column traction-compression machine 3345® (Instron™, Norwood, MA, USA). The protocol involved opening the osteotomy until breakage, defined by the appearance of the first fissure at the hinge. The primary outcome measure was the maximum force at breakage, and secondary outcomes were displacement before breakage and stiffness.

RESULTS

The "Central group" showed, in absolute terms (mean of 130.9 N), the greatest breaking force. This difference was statistically significant (SS) only compared to the "Anterior group" (mean of 85.09 N). The "Central group" also demonstrated the highest stiffness (mean of 15.70 N/mm) with a SS difference compared to the other two groups (9.06 N/mm for the anterior group and 11.98 N/mm for the posterior group).

DISCUSSION: These results confirm the working hypothesis: a centrally positioned pin provides increased protection of the medial hinge during a LOWDFO. Although this in vitro study can only approximate actual in vivo conditions, it provides valuable insights into the optimal position of the pin for hinge protection. However clinical studies are needed.

LEVEL OF EVIDENCE: IV; Experimental study.