The modification and testing of an anatomically shaped radius and ulna interlocking intramedullary nail using statistical shape modelling derived from computed tomography scans.

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Stellenbosch : Stellenbosch University
ENGLISH ABSTRACT: The management of either segmental or comminuted complicated radius and ulna fractures and metastatic disease remains challenging for orthopaedic surgeons. Compression or bridge plate fixation has been insufficient to manage some of these injuries. The advent of locked intramedullary nailing provided a novel way to traverse the entire length of the affected bone and provided an effective treatment option for both simple and comminuted fractures. However, one of the challenges with these devices was the freehand locking of the nondriving end of these nails, which made these systems less popular even though the healing times and union rates were comparable to the plating. The specific issues with locking nails of the forearm relates to the non-driving end locking, where the size of the locking hole is small and can be a challenge to lock, as well as the location of the nail, specifically regarding the proximal radius with the radial nerve in the surgical field. The next issue is the increments of nail length and girth, with nail sizes of 4mm and 6mm diameter and the length increments of 20mm, making the correct nail size choice difficult. The natural curve of the radius needs to be corrected to restore anatomy, but since this is usually a guess related to the opposite side, no approximation has been standardised. The primary basis of design for the intramedullary nail is to establish the correct anatomy of the relevant forearm bones in order to have a reliable measurement to extrapolate the required length, diameter and curvature of the nail. The initial anatomical study allowed us to establish the minimum nail length required by using the shortest radius length as the minimum nail length of 225mm as the shorter bone and the ulna length as the maximum nail length of 265mm as it is the longer bone. The prosthesis was designed with 10mm length increments. The same principle was used to decide on the optimal nail diameter of 4.5mm, with the minimum diameter being 4.87 and 4.53 for the radius and ulna, respectively. Each of the forearm bones has a unique curve with the radius having an arc with a measurable radius of curvature in the medullary canal of 561.93mm ± 93.49 (543.09–580.78). The ulna has a varus angulation of 11.39o ± 3.13 (10.76–12.02). These are measurements that the surgeon can use to bend the prosthesis to the appropriate curve or use the average if this cannot be calculated. The previously mentioned problem with freehand locking of the nail has been that it is technically difficult to perform, and the excessive fluoroscopic radiation exposure required to achieve interlocking. To mitigate this problem in the design, a screw hole was created using arrow slit technology from medieval castles that allow excellent viewing and shooting angles through a small hole. This creates a locking hole with a large aperture on the surface and a 35o approach angle which enables the drill to skirt the edge of the funnel-shaped edge and traverse the hole with minimal problems as the funnel shape is reciprocated on the opposite side. The results from the anatomical study were critical in establishing specific design characteristics of the nail. These results were further used in a linear regression model to predict characteristics for surgical planning or anthropological measurements. A mathematical formula was created from the regression to predict radial length using the ulna length with an 85% (R2 = 0.85) accuracy. The minimum diameter of the radius and ulna canals and the ulna length can be predicted using various measurements inserted into a unique mathematical formula for each. The formula predicts the parameter with an R2 of more than 0.80. These measurements can be used to choose an appropriate nail size for the respective bones. The nail design progressed through computer-aided 3D drawings and 3D printed plastic prototypes with the anatomical measurement data as a guide. Jigs and drill sleeves were designed using the same system, with minor adjustments being made, and the eventual product was created. Medical implant-grade titanium was used for the prototypes, and rough design jigs were also manufactured for testing. A cadaver study was undertaken, emphasising nail fit into the radius and ulna and the ability to achieve interlocking at the non-driving end. This was done by recording the time for the locking screw hole to be drilled and x-ray exposures needed to accomplish this. The locking time is within reason, usually a relatively consistent variable, as the procedure and fracture reduction are typically unique to each trauma. The longest locking time was 112 seconds, and the maximum number of exposures was 12. This compared well to the available literature on the topic. Several minor issues arose during the testing phase with mechanical breaking or wearing of parts on the nail or jig systems. The problems were reported to the engineering teams, and replacements were redesigned and manufactured, which were also tested during the cadaver study phase. The final nail design underwent mechanical testing to evaluate the fatigability of the implant. The process was done to international standards (ASTM F1264-16) and showed that the nail would withstand significant mechanical forces before breaking. The maximum force needed for radius and ulna fixation is 250N, the nail was found to withstand 440N of stress 90% of the time with 1 million cycles applied. The number of cycles relates to the time it will take for the fracture to heal, as it usually requires about three months to accumulate a million cycles with regular activity. This means the nail will withstand most normal physiological forces through movement or exercise. With all the anatomical research, product design, and testing that has been done for this project, the forearm nail that has been produced has passed all the tests and is now ready for clinical testing in trauma patients. The process has now been started to ethically engage in testing this project, which shows the thesis aims have been achieved.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Thesis (PhD)--Stellenbosch University, 2023.