Biomechanical characteristics of calcium phosphate cement in the reinforcement of vertebral pedicle screw fixation

Fengfeng LI; Qinghong ZHANG; Ye HUANG; Yunhua WANG

ABSTRACT

BACKGROUND:

Polymethylmethacrylate (PMMA) can ameliorate the condition between vertebral pedicle screws and peripheral bone-matrix interfaces and notably enhance the strength of screw fixation. However, there are several disadvantages during and after operation such as polymerized thermal damaging effect, toxicity and unabsorbable etc. Calcium phosphate cement (CPC) is biocompatible and biodegradable with good biosafty and produce no heat of polymerization, which is a perfect substitute for PMMA.

OBJECTIVE:

To evaluate the reinforcing effect of CPC on vertebral pedicle screw fixation at biomechanical aspect.

DESIGN:

Randomized control and repetitive observed measurement.

SETTING:

Department of Orthopaedics, Second Affiliated Hospital of Nanjing Medical University.MATERIALS The experiment was conducted in Tongji Medical College of Huazhong University of Science and Technology from August of 2002 to February 2003. ①Two fresh spines from male bodies respectively aged of 52 and 50 years were provided by the Anatomic Department of Tongji Medical College. Ten vertebrae in each spine were obtained (T8-12, L1-5) and taken as 52-year group and 50-year group. Radiographs of these vertebrae were taken to exclude congenital abnormality, fracture, tumor or other pathological changes. Vertebrae in both groups were osteoporoses of grade I and in accordance with experimental requirement.②Main components of solid phase of CPC were micropowder of tricalcium phosphate and tetracalcium phosphate (TTCP) and its main ingredients of liquid phase was citrate solution, which was prepared with solid phase in the ratio of 1g vs 1 mL.Primarily setting-time was 15 minutes and the final setting-time was 12hours with the maximum compressive strength between 45 Mpa and 57 Mpa. ③Diameter of self-made pedicle screws was 5 mm; Length of screw thread segment was 34 mm; Pitch was 2 mm; Depth of screw thread was 0.8 mm.

METHODS:

①Biomechanical test of pedicle screw fixation at final solid time of CPC Vertebrae of 50-year group were taken as testing subjects.Control lateral vertebral pedicle screws were implanted directly in screw path; Strengthening lateral vertebral pedicle screws were inserted after fillingwith CPC. After that, specimens were deposited in a thermostated container for twelve hours at 37 ℃. Maximum axial pull-out strength of vertebral pedicle screw was determinated. ②Biomachanical test of vertebra pedicle screw fixation when CPC primarily hardened specimens in 52-year group were taken as testing subjects. In the same way, vertebral pedicle screw was implanted in the control lateral of vertebral pedicle,while that in the strengthen lateral was implanted after filling of cement,which were placed in the thermostated container for 15 minutes at 37 ℃,the maximum axial pull-out strength of vertebral pedicle in primary setting time were determinated. ③Biomechanical test of CPC in the reinforcement of loose vertebral pedicle screw fixation vertebrae in 50-year group were selected. Loosened vertebral pedicle screws were re-fixed with CPC for 12 hours. Maximum axial pull-out strength of bilateral screws was tested.MAIN OUTCOME

MEASURES:

①Biomechanical testing results of pedicle screw fixation at final solid time of CPC. ②Biomechanical testing results of vertebral pedicle screw fixation when CPC primarily hardened.③Biomechanical testing results of CPC in the reinforcement of loose vertebral pedicle screw fixation.

RESULTS:

①Medians of maximum axial pull-out strength of vertebral pedicle screws in control and strengthening laterals in the 50-year group were 620 N and 1 136 N respectively. Compared with control lateral, that in the strengthening lateral increased by 83 % (P ＜ 0.01). Median of anti-cutting stress increased from 1.16 N/mm2 to 2.13 N/mm2 after being strengthened. ②The medians of those in the 52-year group were 554.5 N and 859.5 N respectively and that in the strengthening lateral increased by 55 % in comparison with that in the control lateral (P ＜ 0.01).The median of anti-cutting stress of reinforced bone-screw interface increased from 1.03 N/mm2 to 1.61 N/mm2. ③Maximum axial pull-out strength of vertebral pedicle screws in control and strengthening laterals in the 50-year group of 12 hours after re-fixation were 517 N and 876 N, which respectively increased by 63.6% or 54.2% (P ＜ 0.01) in comparison with median of that of loose screw in the same lateral.

CONCLUSION:

CPC can enhance vertebral pedicle screw fixation in primary and final setting time, with which loosened screws can be re-fixed.Vertebral pedicle screw in control lateral and strengthening lateral strips from bone-screw interface without peripheral bone and vertebral pedicle being destroyed seriously, which are beneficial to the second insertion of screw.