Relative contribution of the nail and plate to a nail-plate construct for comminuted distal femoral fractures.

The purpose of this study was to assess the biomechanical contributions of the nail and the plate individually to a complete nail-plate construct in the setting of comminuted distal femur fractures. For this biomechanical study, comminuted extra-articular distal femur fractures were created in 24 synthetic osteoporotic femur models. These were then split into three groups: the nail-only group, the plate-only group, and the nail-plate group. After fixation, each specimen underwent sequential axial and torsional loading, and axial and torsional stiffness were calculated and compared. The addition of a nail to a plate-only construct increased axial stiffness by 19.7% and torsional stiffness by 59.4%. The plate-only group and nail-plate group both demonstrated significantly greater axial and torsional stiffness than the nail-only group at all levels of axial and torsional load. (p < 0.001) At 1000 and 2000 N of cyclic loading, the nail-plate group demonstrated significantly greater axial stiffness than the plate-only group (p ≤ 0.018). The nail-plate group demonstrated greater torsional stiffness than the plate-only groups at all levels of torsional loading (p < 0.001). In osteoporotic comminuted distal femur fracture models, most of the axial stiffness in a nail-plate construct comes from the plate. While the combination of the two constructs is not fully additive, the plate contributes the majority of the axial and torsional stiffness in a nail-plate construct. The supplementation of the plate with a nail primarily helps to increase resistance to rotational forces. Level of Evidence: III.

Force Transfer through the Scaphotrapeziotrapezoid (STT) Joint.

Background The scaphotrapeziotrapezoidal (STT) joint transfers forces to the proximal carpal row from the thumb and fingers. Clinically, STT joint osteoarthritis is frequently observed on plain radiographs though its role in the mechanics of the wrist joint remains unclear. Questions/Purposes Our purpose was to use a model of normal wrist types, to predict STT motion upon load. Patients and Methods Five normal computed tomography scans of a wrist type 1 and five wrist type 2 were used to model the wrist. A 200-N force was split and applied to the trapezoid and capitate to replicate forces during a knuckle pushup. The bony movement was predicted by the model as bony movement using finite element analysis. Results We found differences in force transfer through the STT joint between the two wrist types when loading the index and middle fingers. Type 1 wrists moved quantitatively more anterior-posterior, type 2 wrists moved more medially-laterally and more proximally-distally. The trapezium in type 1 wrists moved more in the coronal plane than in type 2 wrists. The trapezoid moved more from distal to proximal in a type 2 wrist, p = 0.03. Conclusion/Clinical Relevance This study found differences in motion upon loading through the STT joint between type 1 and 2 wrists. Type 2 wrists moved more radially toward the proximal scaphoid and scapholunate ligament. This study may provide a mechanical basis for degenerative configurations. By linking observed patterns of degeneration to their mechanical causes we can aid in prevention of arthritis.

Effect of Implant Linkage on Axial and Rotational Stiffness of Nail-Plate Constructs for Comminuted Distal Femoral Fractures.

OBJECTIVES: To determine the effect of linking the intramedullary nail and the laterally applied locking plate to treat comminuted distal femur fractures and allow for immediate weight bearing. METHODS: Comminuted extra-articular distal femur fractures were created in 16 synthetic osteoporotic femurs and split into 2 groups: linked versus unlinked. In the linked construct, in addition to standard plate fixation and proximal locking of the nail, 2 nonthreaded locking bolts (prototypes) were placed through both the plate and the nail. In the unlinked construct, the same number of screws affixed the plate to the bone but were directed around the nail, and separate distal interlocking screws were placed for nail fixation. Each specimen underwent sequential axial and torsional loading, and axial and torsional stiffness were calculated and compared. RESULTS: On average, the unlinked constructs demonstrated a greater axial stiffness at all levels of axial loading, and linked constructs demonstrated greater average rotational stiffness. However, there were no statistically significant differences ( P > 0.189) between the linked and unlinked groups at any axial or torsional load. CONCLUSION: In distal femur fractures with metaphyseal comminution, there were no significant differences in axial or torsional stiffness when linking the plate to the nail. Linking the construct seems to provide no significant mechanical advantage compared with the unlinked environment, but it may be a useful strategy to reduce nail "traffic" in the distal segment with no significant disadvantage.

Functional knee apparatus for the evaluation of ligamentous tensions on contact loads.

BACKGROUND: Soft tissue balancing is integral in providing stability following total knee arthroplasty (TKA). Although intraoperative contact load sensors are providing insights into the effects of soft tissue balancing, there is still a lack of understanding of the relationship between the knee's ligamentous tensions and joint surface contact loads. This study reports on the development of a multifunctional testing apparatus that can quantify the effects of ligamentous tension on joint contact loads in a controlled repeatable environment. METHODS: The functional knee apparatus was constructed to act as an anatomical substitute for the benchtop assessment of intraoperative soft tissue balancing. The system was calibrated through reproduction of results from a cadaveric study that employed intraoperative load sensors. Experimentation was then conducted to quantify the effects of tensile pretension variation on measured contact loads throughout the full range of flexion. RESULTS: A linear relationship between the ligamentous tensions and contact loads was observed, with ligaments contributing to 74-80% of the measured contact loads. Ligamentous tensions could be approximated from measured contact loads to within ± 23 N. CONCLUSION: The proposed apparatus can prove to be a valuable tool in the continued exploration of currently undocumented effects (e.g. surgical alteration) in soft tissue balancing. In addition to quantifying the relationship between ligamentous tensions and joint contact loads, soft tissue loading conditions where bicondylar contact was lost (i.e. known sign of kinematic instability) were identified. As a corollary, this system may be able to provide insights on soft tissue balancing standards predictive of patient outcomes.

Electromagnetic Tracker for Active Handheld Robotic Systems.

We describe the development of the In-Loop Electromagnetic Tracker (ILEMT), designed to meet the demanding latency and resolution requirements for active stabilization of hand motion during precision manipulations such as microsurgery. The prototype surpasses the fastest commercial EM trackers by > 4× in root bandwidth/resolution and 2× in latency. The use of two widely spaced carrier frequencies (e.g., 300 Hz and 10 kHz) enables a particularly simple way of reducing the eddy-current interference caused by nonferrous metals present in the workspace. Previously, metal compatibility has only been achieved at a large cost to measurement speed.