A Biomechanical Comparison of Locked Unicortical with Non-Locking Bicortical Screws for Metacarpal Shaft Fractures in a Randomized Human Cadaveric Model.

Background: Locking plate fixation is widely used in large long bone fixation and is now available for small "long" bones in the hand. Potential advantages of unicortical locked fixation are reduced risk of over-drilling and therefore reduced risk of damage to surrounding structures and reduced risk of irritation from proud screws. Furthermore, unicortical fixation may be used where bicortical fixation is technically impossible. Our aim was to compare fixation strength of unicortical locked plate fixation with bicortical non-locked fixation in a human cadaveric model, by assessing strength under cyclical loading conditions and load to failure (LTF). Methods: 16 matched pairs of embalmed and refrigerated human cadaveric metacarpals were randomly allocated to either unicortical locked or bicortical non-locked plate and screw fixation. A transverse osteotomy was made. Fractures were stabilized with 2.0 mm self-tapping locking or cortical screws. Each metacarpal was then loaded with a 3-point cantilever testing using a 100 N cell on an Instron materials testing device, cyclically loading them at 1,000 repetitions of 30 N and 50 N. If there was no visible failure of the fixation from cyclical loading they were then loaded to failure with a 1 kN cell. Results: There was a significant difference of average LTF between the bicortical non-locking and unicortical locking of 38.07-59.95 N (p < 0.01). However, both groups showed no statistically significant difference when comparing their performance under cyclical loading. Conclusions: The authors regard unicortical locked fixation as a useful adjunct to standard plating technique.

Biophysical characterization of ovine forestomach extracellular matrix biomaterials.

Ovine forestomach matrix (OFM) is a native and functional decellularized extracellular matrix biomaterial that supports cell adhesion and proliferation and is remodeled during the course of tissue regeneration. Small angle X-ray scattering demonstrated that OFM retains a native collagen architecture (d spacing = 63.5 ± 0.2 nm, orientation index = 20°). The biophysical properties of OFM were further defined using ball-burst, uniaxial and suture retention testing, as well as a quantification of aqueous permeability. OFM biomaterial was relatively strong (yield stress = 10.15 ± 1.81 MPa) and elastic (modulus = 0.044 ± 0.009 GPa). Lamination was used to generate new OFM-based biomaterials with a range of biophysical properties. The resultant multi-ply OFM biomaterials had suitable biophysical characteristics for clinical applications where the grafted biomaterial is under load.

Accuracy of computer-assisted navigation for femoral head resurfacing decreases in hips with abnormal anatomy.

Computer-assisted navigation systems for hip resurfacing arthroplasty are designed to minimize the chance of implant malposition. However, there is little evidence computer navigation is useful in the presence of anatomical deformity. We therefore determined the accuracy of an image-free resurfacing hip arthroplasty navigation system in the presence of a pistol grip deformity of the head and femoral neck junction and of a slipped upper femoral epiphysis deformity. We constructed an artificial phantom leg from machined aluminum with a simulated hip and knee. The frontal and lateral plane implant-shaft angles for the guide wire of the femoral component reamer were calculated with the computer navigation system and with an electronic caliper combined with micro-CT. There was a consistent disagreement between the navigation system and our measurement system in both the frontal plane and lateral plane with the pistol grip deformity. We found close agreement only for the frontal plane angle calculation in the presence of the slipped upper femoral epiphysis deformity, but calculation of femoral head size was inaccurate. The use of image-free navigation for the positioning of the femoral component appears questionable in these settings.

Biomechanical evaluation of different designs of glenospheres in the SMR reverse total shoulder prosthesis: range of motion and risk of scapular notching.

BACKGROUND: Reverse total shoulder arthroplasty is a treatment option for cuff tear arthropathy. Scapular notching remains a concern. This biomechanical study compared the range-of-motion in different designs of glenospheres and hence the relative risk of scapular notching. METHOD: A precision coordinate device was used to investigate four different designs of glenospheres (SMR prosthesis); 36 mm concentric (Standard), 36 mm eccentric, 44 mm concentric, and 44 mm eccentric glenospheres. The centre of rotation in each design was first established. The position of the humeral prosthesis was recorded in the plane of the scapula to compare the degree of adduction and the total range-of-motion. RESULTS: Eccentric glenospheres were found to improve range-of-motion by allowing a higher degree of adduction. Larger diameter glenospheres were found to improve range-of-motion by increasing adduction and abduction. Compared to the 36 mm concentric (standard) glenosphere, the 36 mm eccentric glenosphere improved adduction by 14.5 degrees, the 44 mm concentric glenosphere improved adduction by 11.6 degrees, the 44 mm eccentric glenosphere improved adduction by 17.7 degrees. CONCLUSION: Eccentric glenospheres with a center-of-rotation placed more inferiorly were shown to improve adduction. This design may reduce the clinical incidence of scapular notching.

Tensile strength of surgical knots in abdominal wound closure.

Abdominal wound dehiscence is a surgical catastrophe that can be attributed to patients or technical factors. The technical properties of the monofilament sutures and knots that are commonly used in abdominal closure are poorly understood. The aim of this study was to compare the tensile strength of monofilament sutures tied with conventional knots. To do this, the knot-holding capacity of four types of knots (square, surgeons', Aberdeen and loop) were tested using three types of gauge 1 monofilament suture, namely nylon, polyglyconate and polydioxanone, using a tensiometer. We found that the knot-holding capacity of the loop knot was between twofold and threefold greater than all the other knots examined. In comparing suture types, polyglyconate had the highest knot-holding capacity for all the knots that were examined and there was no difference in the tensile strength of nylon and polyglyconate tied in a square, surgeons' or Aberdeen knot (P < 0.05). In conclusion, our findings suggest that closure of an abdominal wound would be best commenced with a loop knot, using gauge 1 polyglyconate and finished with either an Aberdeen square or surgeons' knot would be appropriate.

Orthogonal cutting of cancellous bone with application to the harvesting of bone autograft.

Autogenous bone graft harvesting results in cell death within the graft and trauma at the donor site. The latter can be mitigated by using minimally invasive tools and techniques, while cell morbidity may be reduced by improving cutter design and cutting parameters. We have performed orthogonal cutting experiments on bovine cancellous bone samples, to gain a basic understanding of the cutting mechanism and to determine design guidelines for tooling. Measurements were performed at cutting speeds from 11.2 to 5000 mm/min, with tool rake angles of 23 degrees, 45 degrees and 60 degrees, and depths of cut in the range of 0.1-3.0 mm. Horizontal and vertical cutting forces were measured, and the chip formation process video recorded. Continuous chip formation was observed for rake angles of 45 degrees and 60 degrees , and depths of cut greater than 0.8 mm. Chip formation for depths of cut greater than 1.0 mm was accompanied by bone marrow extruding out of the free surfaces and away from the rake face. Specific cutting energies decreased with increasing rake angle, increasing depth of cut and increasing cutting speed. Our orthogonal cutting experiments showed that a rake angle of 60 degrees and a depth of cut of 1mm, will avoid excessive fragmentation, keep specific cutting energy low and promote bone marrow extrusion, which may be beneficial for cell survival. We demonstrate how drill bit clearance angle and feed rate can be calculated facilitating a 1mm depth of cut.