Osteoporosis has become an important health problem for middle-aged and elderly people. Zachary [15] suggested that the rate of graft subsidence following LLIF is inversely related to bone mineral density. The negative effect in patients with poor bone mineral density has motivated spine surgeons to investigate various pharmacologic methods, such as bis-phosphonates, to maximize bone quality in those kinds of patients. The measurement of preoperative imaging parameters, pedicle screw with vertebroplasty, intraoperative protection of bone endplate and adjustment of bone metabolism postopeartively can effectively improve the imaging results and clinical efficacy after operation.
Since its first introduction in 2012, oblique lumbar interbody fusion (OLIF) has evolved and led to improved outcomes in properly selected patients. Posterior pedicle screw instrumentation was routinely used for stabilization after the OLIF procedure. Traditional bilateral internal fixation is more stable than unilateral fixation in axial rotation and lateral curvature [16]. However, a study [17] has shown that over rigid fixation lead to stress shielding at the fusion segment, resulting in accelerated degeneration of adjacent segments, as well as bone graft absorption of corresponding vertebral bodies. A vitro biomechanical study [18] demonstrated that unilateral pedicle screw fixation combined with a single cage can restore torsional stiffness and other spinal stability indexes. A clinical study [19] showed that unilateral internal fixation decreased the operation time, blood loss and the cost of implants, while no significant difference was found in the clinical efficacy and fusion rate with bilateral internal fixation. Liu et al.[14] performed a prospective study to assess outcomes in 14 patients with lumbar degenerative diseases and reported that patient underwent OLIF and anterolateral screw-rod instrumentation led to a satisfactory effect, which proved that it minimized the operaton time, iatrogenic injuries and medical cost and has the potential to provide efficient spinal segment stability. In this study, a total of 90 patients underwent OLIF combined with anterolateral screw-rod instrumentation and all of them achieved good clinical outcomes at final follow-up.
Cage subsidence is one of the common postoperative indicators that needed to be paid attention. Tokuhashi [21] suggested that the cage subsidence is inevitable on imaging and the subsidence mainly occurred within 1 year after operation, compared with the first postoperative X-ray. The loss of height of intervertebral space of fusion segment was seen at the last follow-up. The degree of cage subsidence increased with time before it achieved spinal stability. Le [22] found that patients with clinical symptoms accounted for 2.1% of imaging subsidence and whether there is a correlation between the cage subsidence and symptoms is unknown. However, severe cage subsidence will lead to fusion failure, spinal sagittal imbalance and Basstrup syndrome [23]. Severe cage subsidence is accompanied by obvious screw loosening and excessive loss of intervertebral space height, resulting in intervertebral foramen stenosis, recurrence or aggravation of low back pain. Zeng et al.[24] performed a study to analyze the early complications OLIF and showed that 18 cases of cage sedimentation or cage transverse shifting in the OLIF stand-alone group. In this study, there were 2 cases of cage subsidence in both groups, respectively. Similar to the normal BMD group, the height of intervertebral space and lumbar lordosis in the osteopenia group were significantly higher than those before operation. At the final follow-up, the height of intervertebral space decreased compared with 3 days after operation, but it was still higher than that before operation, and there was no corresponding clinical symptom occurred. Spinal stability is the main reason of early symptom improvement after surgery, and the poor clinical effect is closely related to cage subsidence. Yang [10] demonstrated that hyperextension of intervertebral space increases the stress of fusion segment, accelerating the occurrence of subsidence and degeneration of adjacent segment. In this study, the disc height is mainly corrected by gradual expansion of the intervertebral space. After inserting the cage, the intervertebral space is pressurized by elastic retraction, avoiding the excessive pressure of intervertebral space and decrease the risk of loss of intervertebral height. According to our experience, we hold the view that the point of vertebral screw placement is the junction of 1/2 and 1/3 of the vertebral body, closing to the upper and lower endplate, where provides the most rigid fixation. There was no significant difference in postoperative lumbar lordosis both in group A and B, compared with that before operation. Correction of the lumbar lordosis will lead to more compression force in the posterior part of the inververtebral disc than in the front of the intervertebral body, and stress will be concentrated in the back of the intervertebral space. The interface between the cage and the endplate is decreased because of excessive expansion of the anterior intervertebral space, increasing the accidence of cage subsidence. For patients with osteopenia, excessive correction of the lumbar lordosis should be avoided. Glassman [20] demonstrated that the positive sagittal balance was the most reliable predictor of clinical symptoms. Higher PT means that the pelvis is in a backward state, increasing the tension of lumbosacral ligaments and muscles, which could be accounted for low back pain. In this study, no significant difference was found in spino-pelvic sagittal parameters between 3 days after operation and the final follow-up (P > 0.05). It demonstrated that anterolateral screw and rod instrumentation maintained spinal sagittal balance both in osteopenia and normal BMD patients. Besides, we found that improvement in sacrum slope (SS) or pelvic tilt (PT) were limited. The poor effect on mismatch of pelvic incidence-lumbar lordosis highlights the limitation of OLIF cage to achieve sagittal balance.
This study had some limitations. The group size was relatively small and therefore this study may be underpowered to detect many changes. The influence of age-related natural degeneration of spine was not excluded. Longer follow-ups are needed to confirm the results we obtained in this study.