Distal radius fracture is one of the most common fractures in older women (∼70,000 cases annually in Canada). Treatment of this fracture has been shifting toward surgery (mainly volar locking plate (VLP) technology), which significantly enhances surgeon's ability to maintain correction. However, current surgical outcomes are far from perfect. There is a need for an implant which maintains the corrected position (reduction), minimizes soft tissue disruption, and is technically easy to perform. A novel internal, composite-based implant was designed to achieve these ends. It is unclear, however, whether this novel implant offers similar fracture fixation as the VLP. As such, the objective of this research was to evaluate the fracture stability (assessed by calculating change in fracture length) of the novel implant and VLP under cyclic fatigue loading. Specimens: Seven radius specimens derived from older female cadavers (mean = 82.3 years, SD = 11.3 years) were used for the experiment. Preparation: A standardized dorsal wedge was removed from the cortex. The distance from the proximal and distal transverse osteotomies was 10 mm and was positioned 20 mm proximal to the tip of the radial styloid. The osteotomy removed all load-bearing capabilities of bone, equivalent to a worst-case-scenario for DRF fixation. Simulated Loading: The proximal end of the radii was potted (fixed) and positioned in a material testing system. To mimic natural loading conditions, hands were cycled between −30°/30° flexion/extension, at 0.5 Hz, for 2000 cycles, while tension load was applied to the tendons (25-N constant force per tendon, 100-N in total). Mechanical testing outcomes: A position tracking sensor used to measure change in fracture length. This change, as a function of number of cycles, was used to assess implant resistance to fatigue loading. Statistical Analysis: A paired student t-test was used to compare the change in fracture length. Level of significance was determined as 5% (p < 0.05). Changes in fracture fracture-length for both the novel implant and plate is shown in Table 1. The paired t-test indicated significant differences between the two groups in terms of change in fracture length (p = 0.026). The outcome of the novel implant ranged from very stable (change in fracture-length = 0.01 mm) to highly un-stable (2.88 mm). We believe the reason for this variance, at least in part, originates from the surgical procedures. Presumably, given that one very strong stabilization (0.01 mm) and one acceptable stabilization (0.37 mm) was obtained, future research directed towards surgical procedures may improve fracture stability. For any figures or tables, please contact authors directly.
Distal radius fractures are the most common osteoporotic fractures among women. The treatment of these fractures has been shifting from a traditional non-operative approach to surgery, using volar locking plate (VLP) technology. Surgery, however, is not without risk, complications including failure to restore an anatomic reduction, fracture re-displacement, and tendon rupture. The VLP implant is also marked by bone loss due to stress-shielding related to its high stiffness relative to adjacent bone. Recently, a novel internal, composite-based implant, with a stiffness less than the VLP, was designed to eradicate the shortcomings associated with the VLP implant. It is unclear, however, what effect this less-stiff implant will have upon adjacent bone density distributions long-term. The objective of this study was to evaluate the long-term effects of the two implants (the novel surgical implant and the gold-standard VLP) by using subject-specific finite element (FE) models integrated with an adaptive bone formation/resorption algorithm. Specimen: One fresh-frozen human forearm specimen (female, age = 84 years old) was imaged using CT and was used to create a subject-specific FE model of the radius. Finite element modeling: In order to simulate a clinically relevant (unstable) fracture of the distal radius, a wedge of bone was removed from the model, which was approximately 10 mm wide and centered 20 mm proximal to the tip of the radial styloid. Bone remodeling algorithm: A strain-energy density (SED) based bone remodeling theory was used to account for bone remodeling. With this approach, bone density decreased linearly when SED per bone density was less than 67.5 µJ/g and increased when it was more than 232.5 µJ/g. When it was in the lazy zone (67.5 to 232.5 µJ/g), no changes in density occurred. Boundary conditions: A 180 N quasi-static force representing the scaphoid, and a 120 N quasi-static force representing the lunate was applied to the radius. The midshaft of the radius was constrained. FE outcomes: To examine the effects of stress shielding associated with each implant, the long-term changes of bone density within proximal transverse cross-sections of radius were inspected. The regional density analysis focused on three transverse cross-sections. The transverse cross-sections were positioned proximal to the subchondral plate, and were distanced 50 (cross-section A), 57 (cross-section B), and 64 mm (cross-section C) from the subchondral endplate. For both implants in all three cross-sections, cortical bone was reserved completely at the volar side. On the dorsal side, the cortical bone was completely resorbed in the VLP model. In all cross-sections, the averaged resultant density was higher for the “novel implant”. The difference ranged from 33% (cross-section A) to 36% (cross-section C) in favor of the “novel implant”. On average, the density values of the novel implant were 34% higher in transverse cross-sections (A, B, and C). This study showed that the novel implant offered higher density distributions compared to the VLP, which suggests that the novel implant may be superior to the VLP in terms of avoiding stress shielding.
Seven knees from seven donors (2M:5F; age:46+/−11) were scanned three times via QCT (GE Lightspeed; BMD Phantom; 0.625x0.625x0.625mm resolution). CTTOMASD regional analyses were performed for medial and lateral compartments; outputting density versus depth plots fit with polynomial regression equations. As density decreases with increased depth from the subchondral surface, a density threshold of 300mg/ cm3 was arbitrarily set to correspond with the safe resurfacing depth. The 300mg/cm3 density threshold corresponds to the average density of subchondral trabecular bone, and is ~2x the density of weak epiphyseal trabecular bone located beneath stiffer subchondral trabecular bone. Precision was defined using coefficients of variation (CV%). In-vivo precision errors associated with CT-TOMASD safe resurfacing depths were less than 2.7%. CV% was 2.7% for the medial compartment depth and 2.6% for the lateral compartment depth. CT-TOMASD demonstrates repeatable measures of safe resurfacing depths invivo. Safe resurfacing depths are measured in relation to defined density thresholds which can be adjusted according to UKA design and patient specifics (e.g., size, sex). CT induces a low radiation dosage due to the low presence of radiosensitive tissues at the knee (~1/10th of a long-leg standing radiograph). CT-TOMASD has potential to be used as a pre-operative imaging technique for improved UKA stability and longevity.
