Simulated Kick Injury to the Mandible in Horses: Study of Fracture Configurations and Physical Parameters of the Impact.

OBJECTIVE: The goal of this study was to generate mandibular fractures in three regions of the diastema using a metal impactor to simulate a kick from a horse and to determine the mean deceleration in the initial phase of the impact event, the maximum contact force, the impact energy necessary to create a fracture and the duration of the impact. STUDY DESIGN: Thirty heads of horses aged between 5 and 20 years and euthanatized for various reasons were used. The heads were attached to a steel bar at the occiput at an axial angle of 45 degrees so that the body of the mandible was positioned horizontally and directly under the trajectory of the impactor. A 2 kg solid impactor was dropped with velocities of 6 to 14 m/s to simulate a kick from a horse. The impact was recorded using a high-speed video camera with a frame rate of 30,000 frames per second. Radiographs of the heads were obtained before and after the simulated kick. RESULTS: Mandibular fractures with configurations similar to those seen in clinical practice were generated at all three locations. The mean deceleration increased with impact velocity and with more cranially located impact positions. Absorbed energy increased with increasing impact velocity when no fracture was generated. CONCLUSION: The susceptibility to experimental fracture of the diastema increased from rostral to caudal locations, which is most probably caused by decreasing mandibular bone strength and an increase in the curvature at the lateroventral aspect of the mandible in that region. Physical parameters depended on fracture occurrence and type.

Effect of two types of shoulder prosthesis on the muscle forces using a generic multibody model for different arm motions.

BACKGROUND: This study aims to analyze the effects of a novel dual-bearing shoulder prosthesis and a conventional reverse shoulder prosthesis on the deltoid and rotator cuff muscle forces for four different arm motions. The dual-bearing prosthesis is a glenoid-sparing joint replacement with a moving center of rotation. It has been developed to treat rotator cuff arthropathy, providing an increased post-operative functionality. METHODS: A three-dimensional musculoskeletal OpenSim® model of an upper body, incorporating a natural gleno-humeral joint and a scapula-thoracic joint developed by Blana et al. (J Biomech 41: 1714-1721, 2008), was used as a reference for the natural shoulder. It was modified by integrating first a novel dual-bearing prosthesis, and second, a reverse shoulder prosthesis into the shoulder joint complex. Four different arm motions, namely abduction, scaption, internal and external rotation, were simulated using an inverse kinematics approach. For each of the three models, shoulder muscle forces and joint reaction forces were calculated with a 2 kg weight in the hand. RESULTS: In general, the maximal shoulder muscle force and joint reaction force values were in a similar range for both prosthesis models during all four motions. The maximal deltoid muscle forces in the model with the dual-bearing prosthesis were 18% lower for abduction and 3% higher for scaption compared to the natural shoulder. The maximal rotator cuff muscle forces in the model with the dual-bearing prosthesis were 36% lower for abduction and 1% higher for scaption compared to the natural shoulder. Although the maximal deltoid muscle forces in the model with the dual-bearing prosthesis in internal and external rotation were 52% and 64% higher, respectively, compared to the natural shoulder, the maximal rotator cuff muscle forces were 27% lower in both motions. CONCLUSION: The study shows that the dual-bearing shoulder prosthesis is a feasible option for patients with rotator cuff tear and has a strong potential to be used as secondary as well as primary joint replacement. The study also demonstrates that computer simulations can help to guide the continued optimization of this particular design concept for successful clinical outcomes.

The Risk of a Shod and Unshod Horse Kick to Create Orbital Fractures in Equine Cadaveric Skulls.

OBJECTIVE: The aim of this study was to compare the potential of an unshod and shod hoof to cause an orbital fracture in the event of a kick. MATERIALS AND METHODS: Thirty-four equine cadaveric orbitae were exposed to a steel or horn impactor in a dropping test set-up. An impactor velocity of 7 m/s was used for both materials. Testing was repeated on the same orbit at a velocity of 10 m/s with the horn impactor if no damage occurred. A high-speed camera was used to analyse the impact process. Physical parameters (peak force and impact duration) were calculated based on quantitative video-tracking. Computed tomographic (CT) scans were generated and fracture configurations described. RESULTS: At 7 m/s, the fracture probability was lower for horn (23.5%) than for steel impactors (70.6%, p = 0.015). On CT-images, damage of the frontal, temporal, zygomatic and lacrimal bones was detected. Furthermore, the orbital socket (17.2%), the supraorbital foramen (34.5%) and the temporomandibular joint (58.6%) were involved. The frequency of affected orbital bones was not significantly different between fractures generated by steel and horn impactors, but the fracture severity was subjectively greater when fractures were generated by steel impactors. CLINICAL SIGNIFICANCE: The orbital fracture probability was significantly higher when a kick of a shod versus unshod horse was simulated. This indicates that keeping horses unshod would decrease the injury risk of neighbouring horses when considering group housing systems.

The influence of aluminium, steel and polyurethane shoeing systems and of the unshod hoof on the injury risk of a horse kick. An ex vivo experimental study.

OBJECTIVES: To evaluate the damage inflicted by an unshod hoof and by the various horseshoe materials (steel, aluminium and polyurethane) on the long bones of horses after a simulated kick. METHODS: Sixty-four equine radii and tibiae were evaluated using a drop impact test setup. An impactor with a steel, aluminium, polyurethane, or hoof horn head was dropped onto prepared bones. An impactor velocity of 8 m/s was initially used with all four materials and then testing was repeated with a velocity of 12 m/s with the polyurethane and hoof horn heads. The impact process was analysed using a high-speed camera, and physical parameters, including peak contact force and impact duration, were calculated. RESULTS: At 8 m/s, the probability of a fracture was 75% for steel and 81% for aluminium, whereas polyurethane and hoof horn did not damage the bones. At 12 m/s, the probability of a fracture was 25% for polyurethane and 12.5% for hoof horn. The peak contact force and impact duration differed significantly between 'hard materials' (aluminium and steel) and 'soft materials' (polyurethane and hoof horn). CLINICAL SIGNIFICANCE: The observed bone injuries were similar to those seen in analogous experimental studies carried out previously and comparable to clinical fracture cases suggesting that the simulated kick was realistic. The probability of fracture was significantly higher for steel and aluminium than for polyurethane and hoof horn, which suggests that the horseshoe material has a significant influence on the risk of injury for humans or horses kicked by a horse.