Locked vs. unlocked plate osteosynthesis of the proximal humerus – A biomechanical study
Introduction
Fractures of the proximal humerus are among the most common injuries. It is a typical injury of elderly individuals with an osteoporotic bone substance. While many of these fractures can be treated conservatively, there is a clear trend towards surgical treatment. This is especially the case in displaced fractures. The main advantage is the possibility of mobilizing the joint at an early stage. Conventional plate fixation has stood the test of time for many years. Besides a possible impairment of perfusion leading to humeral head necrosis (DeFrano et al., 2006), the main problem of plate fixation in the humeral head is the anchoring of the screws in osteoporotic bone. A typical resulting complication is the loosening of the implant with varus dislocation of the head (Hessmann and Rommens, 2001, Lungershausen and Bach, 2003).
Systems with a locked screw plate interface (internal fixators) have been available since 1997 and have reduced the complication rate due to a higher stability. Locked plates are characterized by an angle-stable screw–plate interface, typically realized by threaded screw heads which are either fixed in a thread in the plate (Lill et al., 2004) requiring an exact angle of the screw, e.g., 90°, or by a thread in the screw head cutting itself into the plate in an angle determined by the surgeon (Wolter et al., 1999). Promising clinical results using a locked system in the humeral head were first documented by Wurm et al. (1999). Further clinical studies also reported good clinical results, concluding that the locked plate is now a system of choice in the proximal humerus (Lungershausen and Bach, 2003, Hente et al., 2004, Lill et al., 2004).
In an in vitro study (Lill et al., 2003), different non-locked and locked implants were analysed under static and dynamic varus stress. The humerus T-plate, the cross-screw-osteosynthesis, the unreamed proximal humeral nail (UHN) with spiral cling, the Synclaw proximal humerus nail (SPH) and the locked proximal humerus plate (LPHP) were compared. It was concluded that very rigid implants (HTP, UHN) were the most stable under static tests but showed loosening of the screws under dynamic load. The relatively small anatomically formed LPHP with a locked screw plate interface was found to show the best results in osteoporotic bone anchoring. In addition, combining locked screws with nail implants seemed to be a favorable approach (Mathews and Lobenhoffer, 2004). Thus, it can be concluded from the literature that locked systems are biomechanically favorable in the region of the humeral head.
However, those studies did not directly compare locked vs. unlocked implants of the same shape. Seide et al. (1999) compared equally shaped locked with unlocked osteosyntheses on plastic models of different material densities. It was demonstrated that a locked construction showed significantly more stability in simulated osteoporosis, limited plate to bone contact in tubular bone or short periarticular fragments. For example, in a meta-epiphysial model using foam comparable to cancellous bone a 5-fold increase in failure strength was found for the locked construct.
The hypothesis of a locked plate fixation showing more ultimate strength in the humeral head than a non-locked one was tested in a cadaver specimens study. Care was taken to use the same plate and screw geometry in both groups and to investigate the mode of failure under static as well as dynamic conditions.
Section snippets
Methods
Corresponding left and right humeri were harvested from 12 human cadavers and fresh frozen at −20 °C. Bone density was measured in a CT-scanner (Tomoscan SR 7000, Philips, Eindhoven, The Netherlands) evaluating a circular area in a transverse section at the level of the maximum diameter of the humerus head. The size of the area was adjusted, so that the whole marked region of interest included solely cancellous bone and cortical parts were omitted. Relative values of bone densities were obtained
Static tests under increasing axial load
Analysing the load deformation curves (Fig. 2), at low loads a range of linear elastic deformation was seen. The load deformation curve flattens, while a plastic deformation took place in the bone. The low load elastic stiffness of the construct was 74% higher in the locked group (median 80 N/mm) compared with the non-locked group (median 46 N/mm). Similarly, the linear range until failure was definitely extended in the locked group by 64% (92 N vs. 56 N) (Table 1, Fig. 6). The Wilcoxon paired test
Discussion
In the present study, the hypothesis of a locked plate fixation showing more strength in the humeral head than a non-locked one was tested utilizing human cadaver specimens. As distinguished from comparing different locked and unlocked implants (Lill et al., 2003), the same plate and screw geometry was used in both groups. Regardless of the mineral density, consistent higher failure loads were determined for all locked screw plate specimens. The dynamic study clearly showed a specific mode of
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