We assessed fusion mass quality 1 year after uninstrumented spinal fusion using DEXA scanning. DEXA was chosen as it is the most widely used non-invasive bone quality test used today and has excellent precision with a small radiation dose [35
]. In experimental studies, DEXA has been used to evaluate the bone quality of healing fractures [9
]. These studies have shown acceptable correlation of the DEXA results with mechanical properties of the healing fracture [9
]. One study has also shown that DEXA scanning was able to predict the development of atrophic pseudoarthroses in a canine osteotomy model [30
]. Thus, we felt DEXA could provide some indication of the mechanical status of the fusion mass together with information on the bone status of the fusion mass.
We could not demonstrate any effect of DC stimulation on fusion mass behaviour assessed by DEXA. In the study by Jenis et al. [18
], which demonstrated a positive effect of DC stimulation on bone density, the assessment was performed using image analysis of the X-rays and not by DEXA or any other validated method. Thus, DC stimulation either has no effect or the effect is too small to overcome the influencing factors observed in this study.
We found that the fusion mass quality, measured as BMD, depended significantly on several factors. One of them was age, probably reflecting factors involved with the age-related decline in bone mass seen at all skeletal sites. Also, a linear correlation was found between fusion mass BMD and BMD of the lumbar spine not involved in the fusion. It could indicate that the fusion mass was remodelled to obtain characteristics similar to the spinal bone mass in that particular patient. Thus, each patient might have a “set point” of bone quality at which the fusion mass will end. Experimental studies on fracture healing in rats have investigated the effect of osteoporosis and reported different findings [13
]. Studies looking into long-term changes have shown few differences at early time points, but development of differences in the late periods of fracture healing when remodelling begins to occur [24
]. These and other studies have demonstrated remodelling of the fracture callus towards characteristics equal to those observed in osteoporotic bone. They have coupled the decreased strength of fracture healing in osteoporotic bone to a finding of irregular formation of the trabeculae and less mineral acquisition [13
]. Another factor found to influence the fusion mass negatively was smoking. Smoking has been shown to result in lower BMD, especially in the spine, probably by depressing the vitamin D/PTH system [10
], and it is a well-documented risk factor for lower fusion rates after spinal surgery [5
The amount of graft applied at the operation was linearly related to the fusion mass BMD after 1 year, however with a strong interacting effect of sex, as the relation was far more pronounced in men than in women. Only one study so far has looked into volume changes of the spinal fusion mass. Kim and Ha et al. performed a CT study in which they demonstrated a loss of initial bone graft volume of more than 30% from 2 weeks to 1 year postoperatively in instrumented posterolateral fusions using autograft. From 1 to 5 year postoperatively, the fusion mass volume remained stable. Any effect of gender was, however, not investigated in their study [17
]. The differences between genders observed in our study could be explained by an up-regulation in osteoclast activity in women, due to postmenopausal changes leading to greater bone resorption [21
]. Furthermore, it has been shown that oestrogen can modulate mechano-sensitivity of bone cells [6
]. Thus, oestrogen might play a role via mechanical stimuli or by an effect on resorption in explaining the difference in dependency of graft amount on final fusion mass BMD observed in this study. Another factor could be that men are better adapted to incorporate the fusion mass, as shown by an increased ability of periostal apposition by men compared to women of the same age [40
]. Thus, there simply might be more viable bone cells at the posterolateral fusion bed in men compared to women leading to incorporation of a larger amount of the graft material present. Preparation of the fusion bed was one of the factors we did not control for in this study and it could be thought to vary between surgeons. It has been proven in experimental studies to influence the fusion result.
Another limitation to this study is its cross-sectional design, which prevents conclusions on longitudinal changes and whether the fusion mass has matured. In the only study so far that has applied DEXA for the assessment of fusion mass status after spinal fusion, Libscomb et al. [28
] found that the majority of patients had reached a “steady state” after 1 year, suggesting that the results obtained in this study should be representative of the end status of bony fusion.
We found both the amount of bone graft used as well as spinal BMD to influence fusion rate. In her thesis, Laursen demonstrated that 25 g of autograft per segment included in the fusion seemed to be a critical size to achieve a solid posterolateral fusion as determined from X-rays [26
]. Compared to this, our data suggest that a larger amount of allograft is needed to secure a solid fusion, but also that no “security limit” can be given due to the spread of the data. Regarding spinal BMD, Okuyama et al. [37
] showed patients with a clearly defined fusion after a PLIF to have significantly higher BMD than patients with undetermined union or non-union. They suggested a spinal BMD of 0.674 (±0.104) g/cm2
to be a critical threshold below which non-union starts occurring [37
]. As seen from our result, non-unions can occur well above this threshold in older patients undergoing uninstrumented fusion.
One difference between the assessment of the fusion mass BMD and the CT-based fusion evaluation is that DEXA measures the amount of bone mineral at the fusion site at all levels together and does not reveal anything about the presence of pseudoarthrosis of any kind. Thus, fusions with a significant amount of bone present might be classified as non-unions because of a single transverse line through the fusion mass at one level. Nevertheless significant less bone mineral was present at the fusion site in the non-unions as compared to those solidly fused. This might also explain why the association between fusion mass BMD and functional outcome was weaker than that observed in the primary report between CT-based fusion rate and functional outcome [3
This study demonstrates that the quality of the posterolateral fusion mass is dependant on several factors. Special attention should be given to women and patients with manifest or borderline osteoporosis. Besides optimal preparation of the fusion bed, graft amount should be sufficient when using fresh frozen allograft. But, fusion rates are low and not changeable by DC stimulation using the current magnitudes tested in this study. Other graft materials and stimulatory devices should be tested in a proper fashion in this patient category to prove their efficiency, and the use of autograft might be considered in these patients until more effective fusion methods have been found.