# Rack-and-Pinion Displacement of an Intramedullary Pin During Minimally Invasive Plate–Rod Osteosynthesis of the Canine Femur—A Case Report

## TL;DR

A dog's femoral fracture repair using a common surgical method led to unexpected displacement of an intramedullary pin, likely due to a rack-and-pinion-like effect during screw placement.

## Contribution

This case report identifies a novel failure mechanism during minimally invasive canine femur fracture repair involving intramedullary pin displacement.

## Key findings

- Intramedullary pin displacement occurred during plate–rod fixation due to a rack-and-pinion-like effect from drill bit and screw friction.
- Wear patterns on the retrieved pin were replicated in a bone surrogate model, confirming the displacement mechanism.
- The complication highlights the need for intra-operative implant positioning confirmation during minimally invasive repairs.

## Abstract

Intramedullary pins and plates are commonly used in combination to repair fractures in small animals. This paper discusses a technical complication not seen before, during minimally invasive repair of a femoral fracture in a dog using a common fracture fixation method. Following confirmation of pin placement, placement of the plate and screws resulted in unexpected displacement of the intramedullary pin, necessitating immediate revision. A retrieval analysis of the pin was performed, revealing surface indentations that were characterised and replicated in the laboratory. Displacement of the pin was hypothesised to have occurred due to friction between the drill bit and screws and the pin, causing a “rack-and-pinion”-like effect. The mechanism was replicated using a clinically relevant bone surrogate model. This novel failure method is of importance to veterinary orthopaedic surgeons utilising minimally invasive surgical techniques. By increasing the understanding of the techniques and possible complications that may occur during such repairs, clinical outcomes for patients can be improved.

We present a case of unexpected proximal displacement of an intramedullary pin (IMP) during plate–rod repair of a femoral fracture caused by minimally invasive plate osteosynthesis (MIPO), requiring immediate revision. Implant retrieval analysis and ex vivo modelling were performed to characterise the technique failure mode. The case details are reported. Implant retrieval analysis consisted of stereo zoom microscopic examination of the retrieved IMP. Wear patterns formed by conflict with a 2.8 mm, two-fluted surgical drill bit and a 3.5 mm AO locking screw were replicated using a simple paper impression model. The mechanism of pin movement was replicated in a benchtop laminated polyurethane foam block model, and wear patterns produced during drilling and screw insertion were characterised using stereo zoom. The wear pattern visible on the retrieved IMP suggested axial displacement caused by a rack-and-pinion-like mechanism, enacted by contact with either the drill bit or locking screws during placement of the repair construct. Significant axial displacement of the IMP due to conflict with screws during construct placement is possible during the placement of plate–rod fixation. Surgeons should confirm implant positioning if implant conflict is recognised intra-operatively.