Effect of screw placement order on range of proximal tibial fragment rotation adjustment and osteotomy gap formation when using manual reduction during tibial plateau levelling osteotomy (TPLO).

Aim: To determine the optimal first proximal screw position which permits proximal tibial fragment rotation adjustment while minimising osteotomy gap formation when a manual reduction technique is used for TPLO in dogs.

Methods: TPLOs were performed on bone models using Synthes 3.5-mm TPLO implants with a jig but without the use of an anti-rotational pin. The osteotomy was held in manual reduction with pointed reduction forceps placed across the proximal tibial fragment while the first three screws were applied. The first two screws were placed in the non-locking holes of the distal stem of the plate as per manufacturer's screw placement order guidelines. The third screw was placed in one of the three locking screw positions in the head of the plate, denoted as the 'cranial', 'proximal' and 'caudal' screw positions. After the first three screws were placed, the range of possible proximal tibial fragment rotation change (up to 6 mm in each direction) and the resultant cranial and caudal osteotomy gaps were measured.

Results: The proximal screw position minimises cranial osteotomy gap formation with negative rotation changes to the proximal tibial fragment. The caudal screw position minimises caudal osteotomy gap formation with positive rotation changes to the proximal tibial fragment. Rotation change had a greater effect on cranial osteotomy gaps compared to caudal osteotomy gaps. The cranial screw position had the most limited osteotomy rotation change.

Conclusion: The proximal screw position should be placed first in the head of the plate to allow proximal tibial fragment rotation adjustment while minimising osteotomy gap formation when using a manual reduction technique when performing a TPLO.