Comparative Bending Strength of Metacarpal Neck Fractures Fixed with Two Types of Intramedullary Screws.

Objectives: Intramedullary (IM) screw fixation of metacarpal fractures is a technique, which has gained in popularity owing to its simplicity, speedy rehabilitation, and good functional outcomes. A new, larger diameter, non-compression screw designed specifically for IM metacarpal fixation was recently introduced which could provide better fracture stability and reduce the risk of hardware failure. Our goal was to evaluate the strength of this screw compared to a first-generation screw. Methods: This mechanical study was designed to compare a 4.5 mm metacarpal headless screw (MCHS) to data from our prior research evaluating a 3.0 mm headless screw (HS). Accordingly, we used identical bone models, testing constructs, equipment, and protocols. A metacarpal neck osteotomy was created in 10 Sawbones models. A 4.5 mm x 50 mm MCHS was inserted retrograde to stabilize the fracture. Flexion bending strength was measured through a cable tension construct on a materials testing machine. Failure mechanism and strength was recorded and compared to data with a 3.0 mm screw construct. Results: Eight models failed by bending of the intramedullary screw. Two models failed by rotation of the metacarpal head. Failure occurred at an average of 539 N (Range 315 - 735 N). The MCHS demonstrated a significantly greater load to failure compared to the previously studied 3.0 mm HS at 215 N (P<0.05). Conclusion: A larger, 4.5 mm metacarpal-specific headless screw is more than twice as strong as a 3.0 mm diameter screw in a metacarpal neck fracture model.

A porous swelling copolymeric material for improved implant fixation to bone.

Most metallic commercial bone anchors, such as screws and suture anchors achieve their fixation to bone through shear of the bone located between the threads. They have several deficiencies, potentially leading to failure, which are particularly evident in low-density bone. These include stress-shielding resulting from mechanical properties mismatch; lack of mechanically induced remodeling and osteointegration; and when the pullout force on the anchor, during functional activities, exceeds their pullout strength, catastrophic failure occurs leaving behind large bone defects that may be hard to repair. To overcome these deficiencies, we introduced in this study a porous swelling co-polymeric material and studied its swelling and compressive mechanical characteristics as bone anchor under different configurations. Porosity was achieved by adding a non-dissolvable agent (NaCl) during the process of polymerization, which was later dissolved in water, leaving behind a porous structure with adequate porosity for osteointegration. Three different groups of cylindrical samples of the swelling co-polymer were investigated. Solid, fully porous, and partially porous with a solid core and a porous outer layer. The results of the swelling and simple compression study show that the partially porous swelling co-polymer maintains excellent mechanical properties matching those of cancellous bone, quick swelling response, and an adequate porous outer layer for mechanically induced osteointegration. These suggest that this material may present an effective alternative to conventional bone anchors particularly in low-density bone.

Three-dimensional ankle, subtalar, and hindfoot alignment of the normal, weightbearing hindfoot, in bilateral posture.

The first goal of this study was to develop reliable three-dimensional definitions of alignment for the ankle, subtalar, and hindfoot joints. These alignments are based on three-dimensional morphological features derived from renderings of the bones obtained from weightbearing computer tomography. The second goal was to establish a database quantifying the alignment of the ankle, subtalar, and hindfoot joints in a healthy population during weightbearing bilateral standing. This level 1 study was performed on 95 normal subjects in which random subjects were recruited into a control group. Weightbearing computed tomography scans of the leg were collected in neutral, bilateral, standing posture. In 30 of the subjects, both the left and right leg was scanned. Six alignment parameters for each joint were calculated from morphological measurements conducted on three-dimensional renderings of the bones. Intra- and intertester reliability was assessed from repeated measurements by several testers. Analysis of variance statistics of the alignment parameters showed no statistical differences due to age, gender, or foot side. Intraclass correlation coefficient analysis showed excellent inter- and intratester reliability. It was concluded that the alignment process is comprehensive and reliable. Therefore, without classification by gender or age, it may be used as a foundation for quantifying abnormal alignment associated with various ankle deformities. Clinical significance: The alignment methodology and control database may be used to diagnose ankle, subtalar, and hindfoot misalignment. It can also serve as basis for surgical planning designed to restore normal alignment in various hindfoot pathologies, such as ankle realignment in total ankle replacement.