Load sharing mechanism across graft-bone interface in static cervical locking plate fixation

Abstract

Objective: This study is a retrospective clinical study over more than 4 years of follow up to understand the mechanism of load sharing across the graft-bone interface in the static locking plate (SLP) fixation compared with non-locking plate (NLP).

Methods: Orion locking plates and Top non-locking plates were used for SLP fixation in 29 patients and NLP fixation in 24 patients, respectively. Successful interbody fusion was estimated by dynamic X-ray films. The checking parameters were as follows : screw angle (SA) between upper and lower screw, anterior and posterior height of fusion segment between upper and lower endplate (AH & PH), and upper and lower distance from vertebral endplate to the end of plate (UD & LD). Each follow-up value of AH and PH were compared to initial values. Contributions of upper and lower collapse to whole segment collapse were estimated.

Results: Successful intervertebral bone fusion rate was 100% in the SLP group and 92% in the NLP group. The follow-up mean value of SA in SLP group was not significantly changed compared with initial value, but follow-up mean value of SA in NLP group decreased more than those in SLP group (p=0.0067). Statistical analysis did not show a significant difference in the change in AH and PH between SLP and NLP groups (p>0.05). Follow-up AH of NLP group showed more collapse than PH of same group (p=0.04). The upper portion of the vertebral body collapsed more than the lower portion in the SLP fixation (p=0.00058).

Conclusion: The fused segments with SLP had successful bone fusion without change in initial screw angle, which was not observed in NLP fixation. It suggests that there was enough load sharing across bone-graft interface in SLP fixation.

Keywords: Cervical vertebrae; Spinal fusion; Surgical Fixation Devices.

Fig. 1

Radiological parameters on cervical lateral X-ray film to evaluate changes at the site of interbody bone fusion with anterior cervical plate fixation. ACD & PCD : anterior and posterior cortical distance between upper and lower endplate, SA : Screw angle between upper and lower screws, UD & LD : distance from upper and lower vertebral endplate to the plate.

Fig. 2

Cervical lateral X-ray images of static locking plate (Orion plate) fixation taken immediately (A) and 61 months (B) after surgery. Migration of screws inside the vertebral body with graft collapse is noticed with a minimal change in screw angle. Upper screws migrated more than lower screws.

Fig. 3

Cervical lateral X-ray images of non-locking plate (Top plate) fixation taken immediately (A) and 65 months (B) after surgery. Breakage of screw angle at the plate-screw junction with graft collapse is noticed. Anterior part of fusion segment collapsed more than posterior part.

Fig. 4

Difference between initial and follow-up screw angle. Screw angle of static locking plate showed little change, whereas screw angle of non-locking plate significantly changed at the follow-up (p=0.0067). ^*p<0.05. NLP : non-locking plate, SA : screw angle, SLP : static locking plate.

Fig. 5

Collapsed anterior and posterior heights of fusion segments expressed as percentage to initial measurements. Static locking plate showed evenly decreased fusion height at the anterior and posterior parts, whereas anterior height of non-locking plate showed significantly higher collapse rate than posterior height (p=0.04). ^*p<0.05.

Fig. 6

Percentage contribution of upper and lower portion of graft to whole fusion segment collapse. In static locking plate, contribution of the upper portion to whole graft shortening was significantly higher than in the lower portion (p=0.00058). In non-locking plate, the collapse rate was not different between the upper and lower portion of fusion segments. ^*p<0.05. LD : lower distance between lower endplate and lower end of plate, UD : upper distance between upper endplate and upper end of plate.