Postoperative segmental malalignment after surgery with the Bryan cervical disc prosthesis: is it related to the mechanics and design of the prosthesis?

STUDY DESIGN: In a radiographic study, postoperative segmental alignment was compared between 2 cohorts of 20 consecutive patients operated with a Bryan Cervical Disc Prosthesis. In group 2, patients with severe preoperative kyphosis were excluded for disc replacement surgery and the surgical technique was slightly altered to avoid asymmetric overdrilling of the posterior part of the cranial endplate of the caudal vertebral body. OBJECTIVE: The aim was to investigate whether this change in patient inclusion criteria and modification of the surgical technique had an influence on postoperative segmental alignment and whether postoperative kyphosis is related to the mechanical properties and/or the design of the prosthesis. SUMMARY OF BACKGROUND DATA: Several research groups reported segmental kyphosis after treatment of degenerative disc disease with the Bryan Cervical Disc Prosthesis. METHODS: On the basis of lateral radiographs, the disc insertion angle (as a postoperative estimate for the intraoperative angle of approach) and the angle of the functional spinal unit (FSU) and disc angle (both as measures for segmental alignment) were calculated. RESULTS: In group 1, 80% of the patients had a kyphotic FSU angle and 40% had a kyphotic disc angle preoperatively. At follow-up, 65% of the patients had a kyphotic FSU angle, whereas 55% had a kyphotic disc angle. In group 2, 40% of the patients had a kyphotic FSU angle and 5% had a kyphotic disc angle preoperatively. At follow-up, 40% of the patients had a kyphotic FSU angle, whereas 5% had a kyphotic disc angle. Due to the change in patient inclusion criteria, there was a significant difference in preoperative FSU angle between groups 1 and 2; however, no significant difference in preoperative disc angle was found. Owing to the change in surgical technique, the disc insertion angle was significantly different between both the groups. A difference in postoperative FSU angle, however, nonsignificant, between both the groups was observed. There was a significant difference in postoperative disc angle between both the groups; group 1 showed significantly more kyphosis of the shells, than group 2. CONCLUSIONS: This study shows that segmental malalignment with the Bryan Disc can be reduced and is therefore not device related. Proper patient selection and a modified surgical technique can prevent this adverse outcome.

Histodynamics of bone tissue formation around immediately loaded cylindrical implants in the rabbit.

OBJECTIVES: The local mechanical environment influences early peri-implant tissue formation. It is still unclear whether immediate loading limits or promotes peri-implant osteogenesis and which mechanical parameters are important herein. The present study evaluated the influence of well-controlled mechanical stimuli on the tissue response around immediately loaded cylindrical turned titanium implants at two different observation periods. MATERIAL AND METHODS: A repeated sampling bone chamber, consisting of dual-structure perforated hollow cylinders with a cylindrical implant, was installed in the tibia of 14 rabbits and used to conduct three displacement-controlled immediate loading experiments: (i) 30 microm - 400 cycles/day - 1 Hz frequency - 2 x/week - 6 weeks; (ii) 30 microm - 400 cycles/day - 1 Hz - 2 x/week - 6 weeks, followed by another 6 weeks with a 50 microm - 800 cycles/day - 1 Hz - 2 x/week loading protocol; and (iii) 0 microm implant displacement for 12 weeks. A linear mixed model and logistic mixed model with alpha=5% were conducted on the data set. RESULTS: The tissue area fraction was significantly the highest after 12 weeks of loading. The bone area fraction was significantly different between all three loading conditions, with the highest values for the 12-week loading experiment. Twelve-week stimulation resulted in a significantly higher mineralized bone fraction than 6 weeks. Loading did have a significantly positive effect on the mineralized bone fraction. The incidence of osteoid-to-implant and bone-to-implant contact increased significantly when loading the implant for 12 weeks. CONCLUSION: Immediate loading had a positive effect on the tissue differentiation and bone formation around cylindrical turned titanium implants. Controlled implant micro-motion up to 50 microm had a positive effect on the bone formation at its interface.