MECHANICAL SIGNIFICANCE OF VERTEBRAL BODY OSTEOPHYTES

Abstract

Introduction: Vertebral body osteophytes are common in elderly spines, but their mechanical function is unclear. Do they act primarily to reduce compressive stress on the vertebral body, or to stabilise the spine in bending? How do they influence estimates of vertebral strength based on bone mineral density (BMD)?

Methods: Spines were obtained from cadavers aged 51–92 yrs (mean 77 yrs) with radiographic evidence of vertebral osteophytes (mostly antero-lateral). Twenty motion segments, from T5-T6 to L3–L4, were dissected and loaded a) in compression to 1.5 kN, and b) in bending to 10–25 Nm. Vertebral movements were tracked at 50 Hz using an optical MacReflex system. Bending tests were performed in random order, in flexion, extension, and lateral bending. Resistance to bending and compression was measured before and after surgical excision of all osteophytes. The bone mineral content (BMC) and density (BMD) of each vertebra was measured in the antero-posterior direction, using DXA. Density measurements were repeated after excision of all osteophytes. ANOVA was used to detect changes after osteophyte excision, and regression was used to examine the influence of osteophyte size and BMC.

Results: Removal of osteophytes reduced-vertebral BMD by 9% (SD 13%). Compressive stiffness was affected rather more, being reduced by an average 17% (p< 0.05). Bending stiffness was reduced in flexion and extension by 50% and 39% respectively (p< 0.01), and in left and right lateral bending by 41% and 49% respectively (p< 0.01). Osteophyte removal increased the neutral zone and range of motion in each mode of bending. Most mechanical changes were proportional to osteophyte mass, and to changes in BMC (p< 0.01).

Conclusions: Vertebral body osteophytes primarily stabilise the spine in bending, and do not play a major role in resisting compression. Animal models show that osteophytes grow in response to experimentally-induced instability, so their formation can be seen as mechanically-adaptive (restoring stability) rather than degenerative. The influence of typical osteophytes on compressive stiffness is greater than their influence on vertebral BMD (17% vs 9%) so predictions of vertebral compressive strength based on BMD measurements are likely to be under-estimates if osteophytes are present.