Our method may provide a preoperative tool for the assessment of the local bone quality of the proximal humerus using CT data. Therapeutic adjustments such as augmentation or primary arthroplasty may be considered in patients with very low local BMD.
Introduction Bone strength plays an important role in implant anchorage. Bone mineral density (BMD) is used as surrogate parameter to quantify bone strength and to predict implant anchorage. BMD can be measured by means of quantitative computer tomography (QCT) or dual energy X-ray absorptiometry (DXA). These noninvasive methods for BMD measurement are not available pre-or intra-operatively. Instead, the surgeon could determine bone strength by direct mechanical measurement. We have evaluated mechanical torque measurement for (A) its capability to quantify local bone strength and (B) its predictive value towards load at implant cut-out. Materials and methods Our experimental study was performed using sixteen paired human cadaver proximal femurs. BMD was determined for all specimens by QCT. The torque to breakaway of the cancellous bone structure (peak torque) was measured by means of a mechanical probe at the exact position of subsequent DHS ® placement. The Wxation strength of the DHS ® achieved was assessed by cyclic loading in a stepwise protocol beginning with 1,500 N increasing 500 N every 5,000 cycles until 4,000 N. Results A highly signiWcant correlation of peak torque with BMD (QCT) was found (r = 0.902, r 2 = 0.814, P < 0.001). Peak torque correlated highly signiWcant with the load at implant cut-out (r = 0.795, P < 0.001). All specimens with a measured peak torque below 6.79 Nm failed at the Wrst load level of 1,500 N. The specimens with a peak torque above 8.63 Nm survived until the last load level of 4,000 N. Conclusion Mechanical peak torque measurement is able to quantify bone strength. In an experimental setup, peak torque identiWes those specimens that are likely to fail at low load. In clinical routine, implant migration and cut-out depend on several parameters, which are diYcult to control, such as fracture type, fracture reduction achieved, and implant position. The predictive value of peak torque towards cut-out in a clinical set-up therefore has to be carefully validated.
Antegrade nailing of proximal humeral fractures using a straight nail can damage the bony insertion of the supraspinatus tendon and may lead to varus failure of the construct. In order to establish the ideal anatomical landmarks for insertion of the nail and their clinical relevance we analysed CT scans of bilateral proximal humeri in 200 patients (mean age 45.1 years (sd 19.6; 18 to 97) without humeral fractures. The entry point of the nail was defined by the point of intersection of the anteroposterior and lateral vertical axes with the cortex of the humeral head. The critical point was defined as the intersection of the sagittal axis with the medial limit of the insertion of the supraspinatus tendon on the greater tuberosity. The region of interest, i.e. the biggest entry hole that would not encroach on the insertion of the supraspinatus tendon, was calculated setting a 3 mm minimal distance from the critical point. This identified that 38.5% of the humeral heads were categorised as 'critical types', due to morphology in which the predicted offset of the entry point would encroach on the insertion of the supraspinatus tendon that may damage the tendon and reduce the stability of fixation. We therefore emphasise the need for 'fastidious' pre-operative planning to minimise this risk.
The results suggest that texture information contained in trabecular bone structure visualized on radiographs may predict whether an implant anchorage can be used and may determine the local bone quality from preoperative radiographs.
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