Failure of lumbar spinal fusions can result into adjacent segment disease which may require revision surgery or progress into a sagittal deformity. Adjacent segment degeneration, or adjacent segment disease (when symptomatic), is a well-recognised phenomenon after lumbar fusion. Adjacent segment disease has been described to occur in up to 24%, whereas adjacent segment degeneration is a radiological term and not a well-defined entity with rates of occurrence ranging from 5–90% in the literature.
Several risk factors of adjacent segment disease have been discussed, including age, gender, obesity, pre-existing degeneration, or technical aspects of fusion surgery. Recently, a stronger association between reduced postoperative disc height in the adjacent segment and single nucleotide polymorphisms than fusion has been reported, which indicates that some of the variation in disc height seen is related to natural history and a genetic predisposition. However, risk factors with clear implications on surgical management have not been widely reported.
We have recently found that spino-pelvic malalignment—as described by the difference between the pelvic incidence and lumbar lordosis—increases the risk of revision surgery for adjacent segment disease by a factor of 10, if it is not corrected.1 This became apparent when the spino-pelvic parameters were compared between patients requiring revision surgery and patients not requiring revision surgery matched for age, body mass index and preoperative disc degeneration. Using statistical models, a cut-off value of 10 degrees for the difference between the pelvic incidence and lumbar lordosis could be determined. If it is above 10 degrees, 78% of patients required revision surgery as opposed to only 25% of patients when spino-pelvic alignment was preserved.
It is therefore suggested that restoration of spino-pelvic alignment in short lumbar fusions may reduce the risk of revision surgery for adjacent segment disease. Although prospective clinical data is currently lacking, this finding is further supported by biomechanical data. A patient-specific rigid-body model simulated the different alignments according to the patients’ radiographs.2 This revealed that even in the unfused lumbar spine shear stress was up to 32% higher in all disc spaces and particularly in the prospective adjacent segment after a simulated fusion in the presence of spino-pelvic malalignment. Interestingly, compression forces were also significantly higher, but to a much lesser degree (8%) than shear forces to which the intervertebral disc seems to be less resistant to. Fusion further increased shear stress, but malalignment contributed to a difference only in two-level fusions.
The findings support the clinical observation that patients with a difference of the pelvic incidence and lumbar lordosis are at greater risk for failure. On the other hand, they indicate that even the unfused spine exhibits unphysiologic load transfer across the disc spaces in the presence of spino-pelvic malalignment.
While it appears that the risk of failure of short lumbar fusions increases if the spino-pelvic relationships are not respected and possibly restored, the degenerative lumbar spine with mismatch between the pelvic incidence and lumbar lordosis may exhibit features of a sagittal deformity. Failure of short fusions in this case may extend beyond adjacent segment degeneration and result into decompensation of a latent deformity present in the lumbar spine. The concept of malalignment or mismatch between pelvic incidence and lumbar lordosis therefore connects the degenerative lumbar spine to sagittal deformity.
We have further investigated the relationship of malalignment as the intrinsic and local deformity to global balance.3 The preliminary data presented at EUROSPINE 2015 (2–4 September, Copenhagen, Denmark) showed that in the presence of pelvic incidence-lumbar lordosis mismatch, 60% of the investigated radiographs demonstrated global sagittal imbalance. In other words, 40% were still balanced but when further investigated 61% had a retroverted pelvis in order to compensate for the loss of lordosis. These patients have a compensated and still balanced sagittal deformity which often presents as a latent deformity if only the lumbar spine is considered in isolation.
Our data indicate that in clinical practice, spinal surgeons have to remain vigilant to recognise latent sagittal deformities when investigating the degenerative lumbar spine. The key spino-pelvic parameters such as the difference between pelvic incidence and lumbar lordosis and pelvic tilt should always be measured in the degenerative lumbar spine and if abnormal a whole spine radiograph obtained. The surgical approach should then be chosen according to the alignment goals, even in short lumbar fusions. Restoring the spino-pelvic alignment has not been shown to improve clinical outcome at this stage but the current data suggest that the risk of failure increases if it is not considered.
Dominique A Rothenfluh is a spinal surgeon at Oxford University Hospitals NHS Trust, Oxford, UK
References
- Rothenfluh DA, Mueller DA, Rothenfluh E, Min K. Eur Spine J 2015; 24(6): 1251–8.
- Senteler M, Weisse B, Snedeker JG, Rothenfluh DA. Eur Spine J 2014; 23(7): 1384-93
- Rothenfluh DA, Reynolds J, Dominguez D. Eurospine Annual Meeting (2–4 September 2015, Copenhagen, Denmark; QF #35).
