Fractures to the distal radius are costly and debilitating injuries. While it is generally accepted that the leading cause of these injuries is a fall onto an outstretched arm, the mechanics of the injury are less well understood. The main limitations of past research are the use of unrealistic loading rates or uncontrolled loading protocols. Therefore, the purpose of this research was to examine the mechanical response of the distal radius pre-fracture and at fracture, under dynamic loads indicative of a forward fall. Eight cadaveric radius specimens were cleaned of all soft tissues and potted at a 75o angle (representative of the angle between the volar radius and the ground) up to the distal third of the radius. A custom designed pneumatic impact system was used to apply impulsive impacts to the specimen at increasing energy levels until failure occurred. The intra-articular surface of the radius rested against a model scaphoid and lunate made from high density polyethylene (Sawbones) attached to a 5 degree of freedom load cell that in turn was attached to an impact plate. The position of the carpals within the intra-articular surface simulated 45o of wrist extension. Following failure (defined as the specimen being fractured into at least 2 distinct pieces), the specimens were removed from the testing apparatus and the location, type, pattern and severity of injury was noted and classified using the Frykman and Melone classification systems. Energy input and force variables were also collected at failure.Purpose
Method
Stress shielding (i.e. reduction in bone strains) in the distal ulna is commonly noted following ulnar head replacement arthroplasty. Optimal design parameters for distal ulnar implants, including the length of the stem, are currently unknown. The purpose of this study was to investigate the effect of stem length on bone strains along the length of the ulna. Strain gauges were applied to each of eight cadaveric ulnae to measure bending loads at six locations along each ulna’s length (approximately 1.5, 2.5, 4.0, 6.0, 8.0, and 13.0cm from the ulnar head). The proximal portion of each bone was secured in a custom-designed jig. A materials testing machine applied loads (5–30N) to the ulnar head while native strains were recorded. The ulnar head was removed and the loading procedure repeated for cemented stainless steel stems 3 and 7cm in length, according to a previously reported technique (Austman et al, CORS 2006). Other stem lengths between 3 and 7cm were tested in 0.5cm intervals with a 20N load applied only. Data were analyzed using a two-way repeated measures ANOVA (á=0.05). In general, distal bone strains increased as stem length decreased (e.g. average microstrains at the second distal-most gauges: 138±13 (7cm), 147±15 (6cm), 159±21 (5cm), 186±40 (4cm), 235±43 (3cm)). The native strains were different from all stem lengths for the four distal-most gauges (p<
0.05). No differences were found between any stem length and the native bone at the two proximal-most gauges. The 3cm stem replicated the native strains more closely than the 7cm, over all applied loads (e.g. average microstrains at the third gauge level for a 25N load: 357±59 (native), 396±74 (3cm), 257±34 (7cm)). No stem length tested matched the native strains at all gauge locations. The 3cm stem results were closer to the native strains than the 7cm stem for all loads at gauges overtop of the stem. Overall, the 3cm stem produced the highest strains, and thus would likely result in less distal ulnar bone resorption after implantation. These results suggest that shorter (approximately 3cm) stems should be considered for distal ulnar implants to potentially reduce stress shielding, although this must be balanced by adequate stem length for fixation.
