Effectiveness of Polyaxial Clamp-Rod Internal Fixation Method in Distal Femoral Fractures

Experimental Study in a Sheep Model

Abstract

Background: Distal femoral fractures remain a challenging orthopedic condition due to complex regional anatomy and limited availability of suitable implant options for stable fixation. Optimal stabilization must adhere to biological osteosynthesis principles while maintaining rigid mechanical support - particularly in periarticular fractures. The clamp-rod internal fixation (CRIF) system has previously demonstrated satisfactory clinical outcomes; however, the use of conventional cortical screws may predispose to screw loosening and compromised stability. The modified CRIF (M-CRIF) system, redesigned to incorporate polyaxial locking functionality, has the potential to enhance implant stability and improve healing quality. This study aimed to evaluate the feasibility and clinical performance of the M-CRIF implant compared with the conventional CRIF system in an experimental sheep fracture model.

Materials, Methods & Results: Twelve clinically healthy adult Akkaraman crossbred sheep were randomly assigned to 2 equal groups: Group I (n = 6) treated with standard CRIF and Group II (n = 6) treated with M-CRIF. A standardized supracondylar femoral fracture was created in all subjects, followed by internal fixation according to group allocation. Postoperative monitoring included serial clinical examinations, pain assessment using facial expression and clinical scoring systems, gait and weight-bearing evaluations, vertical force plate analyses, and radiographic examinations on days 0, 3, 7, 14, 28, 45, and 60. On day 60, harvested femurs underwent histopathological assessment of fracture healing characteristics. Intraoperative reduction was achieved in all cases; however, clamp advancement over the bent rod portion was technically more demanding in Group I. Pain scores improved over time in both groups, but Group II showed faster reduction in discomfort, with significant differences observed between early and late time points (P < 0.05). Functional assessments revealed significantly superior gait and weight-bearing in Group II on days 28 and 45 (P < 0.05). Pressure mat data supported these findings, demonstrating higher peak vertical force values in Group II at the same time points (P < 0.05). Radiographic evaluations indicated progressive healing in both groups without significant intergroup differences; however, Group II displayed a more uniform and advanced callus formation pattern. Implant-related complications were limited to 1 case of cortical screw loosening in Group I and one postoperative soft-tissue infection in Group II, the latter resolving without affecting final healing. Histopathological findings revealed nearly complete replacement of cartilage templates by bone, abundant osteoblastic activity, and organized trabecular architecture in both groups, with slightly enhanced tissue remodeling in Group II.

Discussion: The results indicate that incorporation of polyaxial locking screws into the clamp mechanism provides improved construct stability, encourages earlier weight-bearing, and contributes to more consistent functional recovery. Although radiographic outcomes alone were insufficient to discriminate between groups, multidimensional assessment methods - including force plate analysis and clinical scoring - clearly demonstrated the mechanical advantages of the M-CRIF system during the early healing phase. The reduced risk of screw-related complications further supports the superiority of this design in anatomically restricted distal femoral fractures. Considering current trends favoring systems that preserve local vascularity while providing rigid fixation, M-CRIF appears to satisfy both biological and biomechanical requirements more effectively than conventional CRIF.

Keywords: sheep, distal femur, CRIF, locking screw, osteosynthesis, experimental model.