In this paper, we develop a method to build FEA model, boundary conditions and to analyse static strength and rigidity of the object based on the CT patient table. Especially to introduce the examples of how to simplify the actual models to ideal models which can easily add some loads. In the end of the paper, we compare the results analysis of FEA with the results of experiment and we confirm the high accuracy of FEA, so we summarize that FEA technology is very important for the development of medical equipment.
Equipment Design ; Finite Element Analysis ; Models, Theoretical ; Tomography, X-Ray Computed ; instrumentation

Objective:To evaluate the anterior expansion of sacral foramen and decompression of sacral plexus via the lateral-rectus approach (LRA) in the surgical treatment of sacral fractures complicated with sacral plexus injury.Methods:From January 2013 to June 2018, 11 patients were treated at Department of Orthopaedics, The Third Hospital Affiliated to Southern Medical University for obsolete sacral fractures complicated with sacral plexus injury. They were 8 males and 3 females, aged from 17 to 54 years (average, 38 years). According to the Denis classification, all the sacral fractures belonged to Denis Zone Ⅱ. According to British Medical Research Council (BMRC) grading system, the nerve injury was complete damage in 2 cases and partial damage in 9. The mean time from injury to surgery was 6 months (range, from 0.7 to 12.0 months). After the sacroiliac joint was exposed via the LRA, the lumbosacral trunk was exposed and released between iliac vessels and the iliopsoas. Next, the S1 foramen was expanded and the S1 nerve root was released after separation of the median sacral artery and the internal iliac artery. Reduction and fixation of the sacroiliac joint was carried out for patients with unstable sacral fracture. X-ray and CT examinations of the pelvis were performed to evaluate fracture healing and neurological function recovery postoperatively.Results:Of this cohort of 11 cases, operation succeeded in 10 but failed in one whose sacral fracture was found to have completely healed with the S1 foramina totally occluded. The surgical time averaged 110 min (range, from 70 to 220 min) and the blood loss 1, 100 mL (range, from 450 to 2, 800 mL). Postoperative X-ray and CT examinations showed that the sacral foramens were expanded significantly without any complications. The follow-up time averaged 18 months (range, from 12 months to 4 years). By the BMRC grading system at the last follow-up, the neural function was completely recovered in 5 cases, partially recovered in 4 cases and not recovered in one.Conclusion:Significant anterior expansion of sacral foramen and decompression of sacral plexus via the LRA is a viable and effective alternative for treatment of sacral fractures complicated with sacral plexus injury.