Shape memory staples have several uses in both hand and foot and ankle surgery. There is relatively little data available regarding the biomechanical properties of staples, in terms of both the compression achieved and potential decay of mechanical advantage with time. An understanding of these properties is therefore important for the surgeon.

Two blocks of synthetic polyurethane mimicking properties of cancellous bone were fixed in jigs to both the actuator and 6 degree-of-freedom load cell of an MTS servohydraulic testing machine. With the displacement between the blocks held constant the peak value and subsequent decay in compressive force applied by both the smooth and barbed version of the nitinol OSStaple (Biomedical Enterprises), Easyclip (LMT), Herbert Bone Screws (Martin) and the Headless Compression Screw (Synthes) was measured. Nitinol staples were energised once only. A second experiment was conducted to assess the effects of repeated energisation on these parameters.

The Easyclip staples achieved a mean peak force of 5.2N, whilst the smooth and barbed OSStaples achieved values of 9.3N and 5.7N, respectively. The Herbert screws achieved a mean peak force of 9N and the headless compression screws 23.9N. The mean peak force achieved with 2 Easyclip staples in parallel was 8.1N. Following the application of a single energisation the OSStaples exhibited a significant reduction in compressive load, losing up to approximately 70% of the peak value attained. The repeated energisation of these nitinol staples produced progressive increases in both peak and trough loads, the positive effects exhibited a plateau with time.

Performance of both OSStaples was comparable to the Herbert screw with regard to reduction load applied across a simulated fracture plane. The maximum load applied by the OSStaples diminished with time. Staples provide fixation without violating the fracture plane which has the potential to offer some benefits from a healing perspective.

Negative ulnar variance, lunate shape and increased load transmission are associated with Kienbock’s disease. This may reflect trabecular alignment being more susceptible to shear forces along “fault planes” in Type 1 lunates, causing microfractures and avascular necrosis. The aim of this study was to assess the relationship between lunate bone structure, density and ulnar variance.

Standard 90/90 radiographs of 22 cadaveric wrists were taken for ulnar variance and lunate shape. The lunates were harvested and routine CT scans (1mm) were taken in 22/22 in the coronal, sagittal and transverse planes. DICOM files were analysed using Mimics (Materialise, Belgium) to measure Hounsfield units. MicroCT scans (SkyScan, Belgium) (40 μm) were taken in 10/22 in the coronal plane and measured for trabecular angle at the proximal and distal joint surfaces and the ‘tilting angle’ (between scaphoid and radius joint surfaces). Data was anlaysed using one-way ANOVA tests using SPSS for Windows.

Negative ulnar variance was noted in 7/22, neutral 10/22 and positive 5/22. Lunate shape according to Zapico was 0/22 Type 1, 18/22 Type 2 and 4/22 Type 3.

Lunate bone density was significantly lower in the ulnar positive specimens compared to ulnar negative and neutral (p< 0.001) (fig. 1).

The average trabecular angle measured 84.7° (+/− 4.5°) at the proximal and 90.3° (+/− 2.6°) at the distal joint surfaces and tilting angle was 115.7° (+/− 12.0°) (fig. 2). The 50% slice on the microCT correlated best with xray measurements of this angle.

This study quantifies the previous finding that load transmission through the lunate and hence lunate bone density is related to ulnar variance and that this is higher in ulnar negative wrists. MicroCT is a useful modality to assess trabecular structure and supports the ‘fault plane’ hypothesis of Kienbock’s Disease.

The behaviour of two different methods of reattachment of the flexor digitorum profundus tendon insertion was assessed. Cyclical testing simulating the first 5 days of a passive mobilisation protocol was used to compare the micro Mitek anchor to the modified-Bunnell pull-out suture. Twelve fresh-frozen cadaveric fingers were dissected to the insertion of the FDP tendon. The FDP insertion was then sharply dissected from the distal phalanx and repaired using one of two methods: group 1 -modified Bunnell pullout suture using 3/0 Prolene; group 2 micro Mitek anchor loaded with 3/0 Ethibond inserted into the distal phalanx. Each repaired finger was mounted on to a material testing machine using pneumatic clamps. We cyclically tested the repair between 2N and 15N using a load control of 5N/s for a total of 500 cycles. Gap formation at the tendon bone interface was measured every 100 cycles.

No specimens failed during cyclical testing. After 500 cycles, gap formation of the tendon-bone interface was 6.6mm (SD = 1.2mm), and 2.1 mm (SD = 0.3mm) for the pullout technique and the micro Mitek anchor repair respectively. Concerns related to suture anchors, such as anchor failure or protrusion, joint penetration, and anchor-suture junction failure, were not encountered in this study.

Cyclical loading results suggest that the repair achieved with both methods of fixation is sufficient to avoid failure. However, significant gap formation at the tendon-bone interface in the modified Bunnell group is of concern, suggesting it may not be the ideal fixation method.