Tendon Function and Morphology Related to Material and Design of Plates Fordistal Radius Fracture Fixation: a Canine Forelimb Model

Introduction: Substantial disability can result from malunions of the distal radius following fracture. Restoration of accurate anatomical congruity and stable fixation is paramount to obtaining excellent clinical function of articular fractures. Internal fixation is widely accepted as the treatment of choice in achieving this goal. The dorsal radial plate (DRP) pi plate (Synthes USA) was designed for dorsal applications in complex intraarticular and extraarticular fractures of the distal radius allowing multiple points of fixation, smaller implants, bone conforming profile, and periarticular placement to enhance fragment and articular support in highly comminuted fractures. Although the DRP appears to be an optimal implant for the distal radius and good results have been reported, complications such as tendon irritation, adhesions, tenosynovitis, and even spontaneous tendon rupture, have raised issues of concern with surgeons for use of the DRP. In this study, we evaluated the relationship of extensor tendon function and DRP using 2 metal types and 2 plate designs in a canine distal radial model. Methods: The study was composed of 14 dogs weighing 35 kgs studied for 9 mos. under an IACUC protocol. Bilateral distal radial plates were implanted with placement in identical periarticular positions on the dorsal bone surface without fracture. In one group of 7 dogs a standard titanium pi plate (Ti) was placed on one radius and an identical stainless steel plate (SS) on the other, randomized by side. In the other group of 7 dogs the standard titanium pi plate (Ti) was implanted on the right and a titanium pi plate with a ramped profile (Ti ramped) on the left radius. A dorsal approach was used to identify the extensor carpi radialis (ECR) and common digital extensor (CDE) tendons which were elevated and retracted to expose the distal one-quarter of the radius. The primary tendon sheaths of the ECR and CDE tendons were opened along the length of the plate and proximal joint to allow direct contact of the tendons to the plate surfaces. Range of motion measurements (ROM) and orthogonal radiographs were obtained, and then the lower forelimb placed in coaptation bandages for 2 wks. ROM and radiographs were obtained postoperatively and at 3, 6, 9 mos. and of the explanted bones. Mechanical testing was performed on explanted forelimbs obtained from the plate treated radii pairs and non-plated control radii pairs from an additional 5 dogs. Each forelimb was placed in neutral position from which the flexion and extension range of motion (FROM and EROM) were measured. FROM and EROM were determined as the angles from the neutral position to the fully flexed and fully extended, respectively. Flexion and extension motions of the carpus were applied manually to the maximum limits and measured. The ECR and CDE tendons were identified and only exposed distal to the radial articular surface. Tissues over the forelimb were left undisturbed. The excursion limits of ECR and CDE over FROM were labeled with respect to a point on the distal radius and measured with a digital calipers. Insertions of ECR and CDE were resected from metacarpals and the carpus was disarticulated. The proximal radius and ulna were secured to the base frame using PMMA. ECR and CDE tendons were transected 15 mm proximal to the plate to release them from the muscle. Specimens were placed in a saline bath and mounted on the material testing system (Instron). The distal end of ECR and CDE was clamped and pulled separately at a rate 2 mm/min up to 100 mm. Pull out displacement and the resisting forces were measured at 10 Hz, data collected, analyzed, and force-displacement curves generated. Pull out force was determined as the maximum force in the F-d curve. Displacement at force pick-up was determined as the displacement point were the force showed acute increase. The data were analyzed using nonparametric statistics. Results: Dogs in both groups exhibited clinical weight bearing in the coaptation splints, and after splint removal at 2 weeks all dogs resumed and maintained clinical function. All implants remained securely fixed without screw loosening. Thickened tissues were observed over the dorsal distal radius to varying degrees in some limbs in both groups with both metal and plate types, notable by 3 mos. Lameness was not noted in any dog. Range of motion measurements revealed decreased ROM of the carpii in limbs with all implant types from 5 to 65 degrees. This was initially evident at 2 weeks in some limbs and later in others but was not found to be associated with pain. The mean decease in ROM of the SS plated limbs was 20 degrees (range 0-55), Ti plated limbs 16 degrees (range 0-65) & 21.9 degrees (range 0-45), and Ti ramped plated limbs 21.9 degrees (range 0-58) degrees, and were not statistically different Radiographically, periosteal reactions developed along the periphery, over the dorsal plate surface and between the vertical plate arms to in a similar manner in association with both materials and plate designs. A striking histological finding was the thickened extensor tendons and proliferative peritendinous tissues which overlaid the plates. The fascial tendon sheaths were in areas contiguous with peri-implant membrane underlying the plate. In general, the mass of the thickened tissue was due to fibrous infiltration between the collagen bundles within the tendon, and proliferation of synovial and paratenon tissues. Overall, a tendinosis was evident with disorientation of the collagen fibers, fibrillation and fibrocartilage formation of the plate side of the tendons. Scattered sites of hyalinization and cartilage metaplasia were also seen on the plate side of the extensor tendons. These features were found in the ECR and CDE extensor tendons over both plate designs and with either metal type. However, they were not specific to any one design or metal type but seemed to occur with similar frequency in all three plated limb types. Mechanical testing revealed that excursion and displacement at force pick up was not different among the groups.