Introduction: Compression staples are becoming increasingly popular for osteotomies and arthrodesis. Their design can be divided into “Mechanical Compression” or “Shape Memory”. However, there are no publications investigating the actual compressive forces achieved or the ideal limb-length to staple width ratio.

Methods and Materials: Compression was compared using a load cell mounted within a previously validated simulated fusion site. Two designs each of “mechanical compression” and “shape memory” staples were tested and filmed. The effect of altering limb length on compression was noted.

Results: Both designs of “mechanical compression” staple splayed open causing either no net compression or even distraction. Distractive forces of up to 23N were recorded. The “shape memory” staples all achieved compression at the fusion site of between 5 and 25N. Limb length did not appear to alter the compression force achieved. The outcome was not affected by the material used.

Discussion: “Mechanical compression” staples act in a similar manner to the AO principle of a 2-hole compression plate used without a lag screw or pre-bending. Although there is compression of the cis-cortex, the limbs of the staple splay open with a fulcrum around the bridge-limb intersection resulting in distraction of the trans-cortex. “Shape memory” staples compress both the cis-and trans-cortices along the length of the limb leading to adequate stability and compression forces across the fusion site.

Conclusion: “Mechanical compression” staples cause a distractive force rather than a compressive force and we therefore recommend that they are not relied upon for fusion and the manufacturers need to modify the product or it’s indications for use. The “shape memory” staples do provide compression and the length-to-width ratio of the staple does not appear to be important.

Total hip replacement in patients with advanced osteonecrosis of the femoral head is often complicated by early loosening of the femoral component. Recent evidence has suggested that abnormal bone extending into the proximal femur may be responsible for the early failure of the femoral component. We aimed to identify which patients were at high risk of early failure by evaluating gadolinium-enhanced MR images of histologically-confirmed osteonecrotic lesions beyond the femoral head.

Although the MR signal intensity has been shown to correlate well with osteonecrosis in the femoral head, it was found to be relatively insensitive at identifying lesions below the head, with a sensitivity of only 51% and a predictive value of a negative result of only 48%. However, the specificity was 90%, with the predictive value of a positive MRI finding being 86%. Only those patients with osteonecrosis of the femoral head secondary to sickle-cell disease, who are known to be at high risk of early loosening, had changes in the MR signal in the greater trochanter and the femoral shaft. This observation suggests that changes in the MR signal beyond the femoral head may represent osteonecrotic lesions in areas essential for the fixation of the femoral component. Pre-operative identification of such lesions in the neck of the femur may be important when considering hip resurfacing for osteonecrosis of the femoral head, following which early loosening of the femoral component and fracture of the neck are possible complications.