A considerable number of patients, especially young patients, complain of lumbar pain or have bilateral lumbar spondylolysis during physical examination, without or only mild lumbar spondylolisthesis, and usually have no neurological problems in imaging, symptoms, and signs(15). However, there is no consensus or guideline for the treatment of this part of patients. The treatment of simple bilateral lumbar spondylolysis can be divided into conservative treatment and surgical treatment(16). The current clinically accepted view is that active surgical treatment should be adopted for patients who are ineffective in conservative treatment for 6 months and are in the terminal stage of isthmic fissure on imaging(4, 17).
Previous studies have revealed a great number and variety of surgical techniques for the spondylolysis repair, demonstrating the lack of consensus on a satisfactory procedure. It mainly includes inter-segmental fusion and intra-segmental isthmus repair(18). For patients with simple lumbar spondylolysis, fusion has a definite effect on the fixation of the vertebral body, effectively preventing the diseased vertebrae from further spondylolisthesis, however, it will sacrifice the mobility of the fused segment, and may accelerate the degeneration of the adjacent segments of the vertebral body and intervertebral disc; while the intra-segment repair is considered to preserve the anatomical structure(19). With the improvement of surgical technique and material development. The method of fixing the diseased segment by direct repair surgery gradually develops from the early isthmic lag screw(20), transverse process-spinous process wire(21), screw-hook construct(22) to the shaped rod and combination techniques. However, each of these approaches has its drawbacks. The isthmus is too small to choose a suitable lag screw, and the lack of bone grafting in the isthmus leads to difficulties in bone healing. The wiring technique requires greater surgical exposure, with extensive stripping of the muscle in order to expose the transverse process completely. The uneven force on the bilateral transverse processes can lead to complications such as transverse process fractures and wire loosening, which may lead to nonunion of the pars defect. Gillet et al.(23) firstly use "V" rods to connect bilateral pedicle screws instead of laminar hook fixation. Ulibarri et al.(24) used this improved method to implant bilateral pedicle universal screws to connect the bent "U"-shaped titanium rod have achieved a satisfactory fusion rate(25). Although the hook-screw and shaped rod methods have achieved satisfactory results, a considerable number of spondylolysis patients are associated with laminosis or dysplasia of spinous processes, which may affect fixation strength.
In this study, the pars defect repair we propose using inter-segmental pedicle screws seems to be a technically simple and safe procedure that presents the advantage of placing the spondylolysis under strong compression to help ensure fusion. Unlike other techniques, which demands extensive muscle stripping, exposing the transverse process, injury of the interspinous ligament during the procedure. Easy surgical access through the Wiltse approach allows minimal soft tissue dissection and reduced blood loss. Since hyperextension and rotation are the main stresses in the fatigue fracture of the isthmus, these above-mentioned various methods of inter-segment fixation have a weaker ability to resist rotation and extension, and the inter-segment fixation provides stronger stability. In addition, the connecting of two screws allows for progressive compression across the isthmus, which increase the degree of bony contact to promote higher fusion rate. Otherwise, for cases with spondylolisthesis within grade 1 and mild disc degeneration, motion segment fixation can correct and stabilize the spondylolisthesis. Compared with the method of inter-segment fixation, its disadvantage is that the original motion segment is fixed. Therefore, in order to restore the motion of this segment, the patients in our study have been removed the internal fixation after CT confirmed that the pars have completely union.
Facet joints and discs are both involved in stability of the lumbar spine construct(26, 27) and considered the relationships among facet joints and discs in adjacent segment degeneration(28, 29). To our knowledge, the incidence and risk factors of fixed and adjacent segment degeneration after inter-segment pedicle screw fixation for lumbar spondylolysis, have not been previously investigated. In this study, we identify the risk factors for facet joint and disc degeneration in order to predict and prevent this condition. After the follow-up on 54 patients, 100% bone healing were achieved. Only one patient had a reduction in disc height of ≥ 20% in the upper and fixed segments, respectively. When the internal fixation is removed,we found a 9% incidence of disc degeneration for 1 grade at the L4/5 and L5S1 levels, respectively (based on the Pfirrmann 5-grade classification system). However, no previous studies compare the development of adjacent and fixed disc degeneration in the patients treated with inter-segment pedicle screw fixation. Previous studies reported adjacent biomechanical alterations after lumbar fusion. Umehara et al.(30) reported that the load burden and weight shearing of the posterior column increased significantly at the adjacent segments. Weinhoffer et al.(31) also reported a significant increase in the disc pressure in the levels above the fused segments. In a systematic review, Harrop et al.(32) reported a 9% incidence of ASD after total disc replacement and a 34% incidence after fusion. We also found a 15% incidence of disc height decrease more than 10% at the L4/5 level and 17% incidence at the L5S1 level, however, the differences were not significant between pre- and post-surgery. The incidence of facet joint degeneration was 13% at the L4/5 level and 17% at the L5S1level, respectively. Facet joint degeneration may arise from different mechanisms. Firstly, surgical factors such as damage to the articular process during screw placement may lead to an increased risk of facet joint degeneration. Increased loading of the adjacent level after fixation also increases the load on the surrounding facet joints(33, 34). Biomechanical studies have shown that the facet load can increase at the level of surgery after intervertebral fusion. Although there is no intervertebral fusion in this study, inter-segmental fixation may also increase the load on the facet joint.
This result of present study suggests that the duration of fixation as a risk factor associated with the occurrence of facet joint degeneration. In these patients, fixed time is considered as a significant factor for facet degeneration. The non-degeneration cut-off value of fixed time has been speculated to be within 15.5 months. As a matter of fact, patients in this series with successful pars union presented a mean value of fixed time of 17.04 months. Therefore, regular postoperative follow-up is required to determine the bony union of the pars defect. Once bony union is achieved, remove of the internal fixation should be recommended. In addition, bone morphogenetic protein-2 (BMP-2) can be used to achieve earlier and enhance fusion. The earlier the bony union occurs, the lower the incidence of facet joint degeneration. Secondary, our results have shown that older age and higher BMI are associated with the occurrence of facet joint degeneration at the L4/5 and L5S1 levels, respectively. In addition, Sagittal balance and spino-pelvic parameters also probably influence facet joint degeneration development. Further study is needed to analyze spinopelvic parameters.
This study has some limitations. First, it was a retrospective study and was not performed as a comparative study. Second, the sample size in the present study was limited. It may increase the chances of making a type II error. Third, radiologic evaluations, including sagittal alignment assessment, were not performed.