March 2012 - Institute of Physics and Engineering in Medicine

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SCOPE | FEATURE ▼ FIGURE 1. Description of the method. ▼ FIGURE 2. Reference points for the PHILOS plate and the nonfixated humerus. ▼ mineral density, bone tissue mineral density and bone volume fraction, while BMA is defined by trabecular thickness, trabecular number, trabecular separation, degree of anisotropy and structural model index. This method assesses the BMD and BMA in the cancellous bone regions where the anchoring elements of an implant (in our study, the locking screws) are to be placed, before the actual fixation is carried out. By identifying the directions in bone where the best purchase can be achieved, the implant design can be optimised. The method is applicable to any locking implant, and we have tested its feasibility on a locking proximal humeral plate available on the market. METHOD The method consists of four steps: (1) temporary, partial fixation of the plate to the humeral shaft using locking screws (outside the region of interest) in order to define a reference system, (2) implant removal and high-resolution peripheral quantitative computed tomography (HR-pQCT) of the bone, (3) determination of BMD and BMA along the implant-anchoring locations (within the region of interest) and (4) evaluation of alternative, optimised directions for implant-anchoring locations (figure 1). FIGURE 3. Implant optimisation procedure by assessing the local bone properties along alternative paths for each screw. ▼ Reference system definition We defined two co-ordinate systems, one for the plate and one for the specimen. Two sets of three unaligned reference points on the corresponding 3D reconstruction were retrieved (figure 2). For the plate, the first reference point (A) was the most distal locking hole, the second reference point (B) was the second most distal locking hole and the third reference (C) point was the tip of the locking screw in the second most distal hole. For the bone, the first reference point (A) was the most distal hole on its medial side, the second reference point (B) was the second most distal hole on its lateral side and the third reference point (C) was this same hole on its medial side. Virtual screw position and evaluation of peri-screw bone quality The co-ordinates of the proximal screws were collected from the absolute co-ordinate system of the 10 | MARCH 2012 | SCOPE
FEATURE | SCOPE plate 3D model. Given the position and direction of the screws in the plate 3D model, a transformation matrix was defined to determine the virtual position and direction for each proximal screw in the 3D model of the humerus. The intact humeral head was aligned to each calculated virtual screw direction, allowing the volume of interest (VOI) around each screw to be defined. Bone parameters were assessed in a cylindrical VOI (25 mm in length, 6 mm in diameter). Optimisation procedure We investigated all the bone quality parameters (BMD and BMA) along four alternative directions. The head of the screw was always kept at the same location while the tip was displaced either anteriorly (Ant), posteriorly (Pos), proximally (Pro) or distally (Dis) (figure 3). For each proximal PHILOS plate screw, the best direction was calculated. Peri-screw bone distribution The method was used to assess differences in peri-screw bone distribution for a population of 19 humeri. 10 BMD was determined in the exact locations where the six proximal screws would have been positioned after completing the instrumentation. Univariate analysis of variance weighted for age was used to compare the different positions with respect to BMD. RESULTS The method described here was able to find the position and direction the real screws would occupy in the humeral head. The optimisation carried out on the cadaveric specimen showed the possibility of finding combinations of new screw directions characterised by better bone properties compared to those of the actual directions. We chose the best screw directions based on the apparent density and bone volume fraction, as they are the most representative indicators of bone strength 11,12 and the most variable parameters for the alternative paths. The effect of the method’s accuracy was found to be negligible when compared to the variability of the parameters obtained from the optimisation procedure. Significant differences in BMD were found between screw positions. In particular, the posteromedial part of the humerus was found to have a significantly higher bone quality. These results prove that different screw configurations could have an effect on bone strain distribution and magnitude during loading. SUMMARY We developed a novel method to investigate peri-implant bone quality distribution. The method can be used to optimise the design of locking implants. It defines the optimal, mechanically most valid paths for the anchoring elements based on local bone properties. The novel design must still take into consideration the surgical approach and the anatomical structures (e.g. soft tissue dissection, blood vessels and nerves). The strengths of the method are: (1) the resolution of the performed scans which allows the analysis of bone quality parameters; (2) the complete lack of metal artifacts, and (3) the possibility of accurately investigating local bone properties while taking into consideration the implant geometry and fixation technique. The goal is to design implants that better fit and take advantage of local bone characteristics in osteopaenic and osteoporotic patients. There are also some limitations to the method. It is based on the assumption that surgeons will position the same implant in different patients similarly, using a standard surgical access. The method also assumes that better mechanical properties of the bone-implant construct are achieved by aiming the implant-anchoring parts at bone regions with proven higher BMD and better BMA. Different screw configurations were found to have an effect on bone strain distribution and magnitude during loading, but additional mechanical testing on paired cadaveric bones and in vivo clinical trials are mandatory to show that implants optimised using this method offer better mechanical performance in fracture fixation. In conclusion, this study has shown that screw paths encountering better purchase can be identified. The resulting optimised implant can potentially better distribute the stress on cancellous regions, and thus help surgical intervention in reaching better clinical outcomes with less post-operative complications. n REFERENCES 1 Südkamp N et al. Open reduction and internal fixation of proximal humeral fractures with use of the locking proximal humerus plate. Results of a prospective, multicenter, observational study. J Bone Joint Surg Am 2009; 91: 1320–28. 2 Sanders BS, Bullington AB, McGillivary GR, Hutton WC. Biomechanical evaluation of locked plating in proximal humeral fractures. J Shoulder Elbow Surg 2007; 16: 229–34. 3 Tingart MJ, Lehtinen J, Zurakowski D, Warner JJ, Apreleva M. Proximal humeral fractures: regional differences in bone mineral density of the humeral head affect the fixation strength of cancellous screws. J Shoulder Elbow Surg 2006; 15: 620–24. 4 Hernandez CJ, Keaveny TM. A biomechanical perspective on bone quality. Bone 2006; 39: 1173–81. 5 Hernandez CJ. How can bone turnover modify bone strength independent of bone mass Bone 2008; 42: 1014–20. 6 Kozic N et al. Optimisation of orthopaedic implant design using statistical shape space analysis based on level sets. Med Image Anal 2010; 14: 265–75. 7 Nobari S, Katoozian HR, Zomorodimoghadam S. Three-dimensional design optimisation of patient-specific femoral plates as a means of bone remodelling reduction. Comput Method Biomech 2010; 13: 819–27. 8 Wirth AJ et al. Implant stability is affected by local bone microstructural quality. Bone 2011; 49: 473–8. 9 Schiuma D, Brianza S, Tami AE. Development of a novel method for surgical implant design optimization through noninvasive assessment of local bone properties. Med Eng Phys 2011; 33: 256–62. 10 Brianza S, Röderer G, Schiuma D, Schwyn R, Scola A, Gebhard F, Tami AE. Where do locking screws purchase in the humeral head Injury 2011. 11 Griffith JF, Engelke K, Genant HK. Looking beyond bone mineral density: imaging assessment of bone quality. Ann NY Acad Sci 2010; 1192: 45–56. 12 Huber MB et al. Proximal femur specimens: automated 3D trabecular bone mineral density analysis at multidetector CT – correlation with biomechanical strength measurement. Radiology 2008; 247: 472–81. ▼ SCOPE | MARCH 2012 | 11
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March 2012 - Institute of Physics and Engineering in Medicine

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