Isometric points in cerclage for patellar tendon repair: a 3-dimensional, weightbearing computed tomography kinematic simulation.

PURPOSE: The gold standard for the management of ruptured patellar tendon is the surgical repair with nonabsorbable sutures and additional reinforcement e.g. cerclage wires as described by McLaughlin in 1947. Despite the early description, the exact placement of such cerclage wires remains unclear. The aim of the study was (1) to analyze how the cerclage elongates during knee flexion and (2) to define the insertion points with the most isometric behavior at the patella and tibial tuberosity (TT).

METHODS

3D simulations of 10 healthy knees in the range from 0° to 120° of flexion using weightbearing computed tomography scans were generated. 12 insertion points were defined at the patella in equal distances along the lateral and medial border from the proximal to the distal pole. 42 insertion points were defined at the tibial tuberosity (TT) in a 3.0 × 3.0 cm large grid around its center. An automatic string simulation algorithm was used to calculate the length for all possible combinations of the cerclage from 0° to 120° flexion. The most isometric combination of insertion points was found using the isometric score.

RESULTS

The most isometric insertion at the TT is distally (p < 0.01) and towards the center of the TT (p < 0.01). The most isometric points at the TT is located 61 mm distal to the joint line and 5 mm from the anterior border of the TT in a true lateral view fluoroscopy. The insertion on the patella is insensible, as long as the patella is not bypassed proximally (p < 0.01).

CONCLUSION

Elongation of patella tendon augmentation or cerclage can be reduced when choosing a distal and central insertion at the TT, while the insertion on the patella does not affect elongation as long as the patella is not by passed proximally. This might help to reduce excessive stress and prevent cerclage or augmentation failure.