Influence of bone mineral density on pedicle screw fixation: a study of pedicle screw fixation augmenting posterior lumbar interbody fusion in elderly patients.

BACKGROUND CONTEXT

Some biomechanical studies have demonstrated that bone mineral density of the lumbar spine (BMD) affects the stability of pedicle screws in vitro.

PURPOSE

To investigate influence of BMD on loosening and related failure of pedicle screws in vivo.

STUDY DESIGN/SETTING: A clinical study of 52 patients who underwent pedicle screw fixation augmenting posterior lumbar interbody fusion (PLIF).

PATIENT SAMPLE

There were 13 men and 39 women, with an average age of 63 years (range, 45-76 years) at the time of operation. The mean follow-up period was 2.8 years (range, 2-6 years).

OUTCOME MEASURES

Relationship between BMD, screw loosening, and its related failures were statistically analyzed.

METHODS

BMD was measured by the dual energy X-ray absorptiometry (DEXA) method. Radiographic assessments were done by the first author and independently by another orthopedist who was not informed of the values of BMD.

RESULTS

The mean BMD of all patients was 0.879 +/- 0.215 (mean +/- S.D.) g/cm2. The mean BMD in patients with and without screw loosening was 0.720 +/- 0.078 g/cm2 (n=11) and 0.922 +/- 0.221 g/cm2 (n=41). There was a significant difference between the mean BMD of patients with and without screw loosening (P<.01). The mean BMD of patients with "union," "nonunion" and "undetermined union" was 0.934 +/- 0.210 g/cm2 (n=40), 0.674 +/- 0.104 g/cm2 (n=4) and 0.710 +/- 0.116 g/cm2 (n=8), respectively. The mean BMD of patients with "union" was significantly greater than those with "nonunion" and "undetermined union" (P<.05).

CONCLUSION

It could be concluded that BMD has a close relation with the stability of pedicle screws in vivo, and BMD value below 0.674 +/- 0.104 g/cm2 suggests a potential increased risk of "nonunion" when pedicle screw fixation is performed in conjunction with PLIF.