The results of instrumentation removal following posterior spinal fusion for treatment of AIS remain controversial. Rathjen et al.  studied 43 patients who had undergone posterior spinal fusion for idiopathic scoliosis and subsequently had complete removal of all instrumentation. They reported two patients with a progression of their coronal curve exceeding 10° (11° and 20°). Potter et al.  reported an immediate loss of approximately 4° (range 0°-8°) after removal, with continued settling of an additional 6° (10° total) in the main thoracic curve in adolescent idiopathic scoliosis. Helenius et al.  reported on 78 patients with adolescent idiopathic scoliosis after Harrington Instrumentation. Fifty of these patients underwent routine rod removal two years after spinal fusion. The mean preoperative, two-year follow-up, and 20-year follow-up thoracic Cobb angles were 53°, 40°, and 48°, respectively, in the patients who had implants removed. Muschik et al.  reported that radiographic follow-up revealed significant loss of correction versus the measurements obtained immediately before instrumentation removal in all 45 patients. Mean correction loss was 6° (from 31° to 37°) for the primary (thoracic) curve. In contrast, 15 months after removal, the main thoracic curve in the present patient increased from 29° to 57°. Surprisingly, her coronal curve fifteen months after removal was just as large as her initial, preoperative one (56°). This amount of progression far exceeds the reported data in both magnitude and rapidity of curve progression.
Several possible explanations could account for the amount of coronal curve progression following implant removal. Preoperative CT prior to reinstrumentation revealed partial clefts between T11 and T12, but no clear evidence of a pseudarthrosis. The partial cleft in the second reinstrumentation surgery showed one potential pseudarthrosis that had not been recognized at the time of surgical exploration during the removal surgery. The patient additionally might have had other occult pseudarthroses, despite the thoracic curve having demonstrated minimal flexibility within the fused area. Initially, the pseudarthrosis might have been covered by a weak sheet of bone similar to a mature fusion bone such that no motion was identified within the fused segments after removal. If we had subsequently removed the weak sheet of bone more, the pseudarthrosis might have manifested. Deckey et al.  reported that surgical exploration is imperfect for positive detection of a pseudarthrosis. CT, flexion/extension radiographs, and/or bone scintigraphy may be helpful to confirm the location and morphology of established nonunions, but current radiographic techniques are also inadequate for reliably excluding the presence of pseudarthrosis . In previous reports, most of the authors identified no patient who was found intraoperatively to have a pseudarthrosis. The cleft in our case is not incompatible with these reports: because there is no definitive method for detecting pseudarthrosis, some of the patients in these prior studies may have had occult pseudarthrosis that was not detected. To our knowledge, no prior published reports have described a rapid worsening of the coronal curve in AIS as severe as that observed in the present patient. One possible explanation is that our patient might have had undetected pseudarthrosis covered by a weak sheet of bone similar to a mature fusion bone. Even if our patient had undetected pseudarthrosis, the observed amount and rapidity of curve progression is still unusual. Lack of good quality and quantity of autologous bone at the first surgery might have affected the lack of spinal fusion. Alternatively, the amount of fusion in our patient may have played a role in the coronal curve progression. Compared to pedicle screw fixation, which has been widely used in recent years, upper hook instrumentation with autogenous rib and local bone grafting might not have been strong enough to produce solid bone union. The thin layer evidently had enough strength to prevent mobility, requiring an osteotomy to correct, but possibly its strength was insufficient to resist the continuous load placed upon the spine, leading to coronal progression. Moreover, partial facetectomy in the first surgery might have created conditions resulting in worse effects compared with the natural history of uninstrumented scoliosis. Potter et al.  suggest that some differences between patients in curve settling reflect the degree of fusion mass consolidation or maturation. Usually, the increase in curvature that occurs is well-tolerated clinically , but in our patient who developed the worst reported increase in curvature, the coronal and sagittal curves became symptomatic, and notable cosmetic deformity recurred. One possible mechanism of the correction loss is that rebalancing the shoulders by a postural control system might increase the left proximal thoracic curve and secondarily the right thoracic main curve. This is speculation, however, and further study is required to establish the pathogenesis.