Early fracture of the sacrum or pelvis: an unusual complication after multilevel instrumented lumbosacral fusion

STUDY DESIGN: A retrospective review of a series of cases with a complication of instrumented lumbosacral fusion. OBJECTIVES: To present a previously undescribed complication, early sacral or pelvic stress fracture, after instrumented lumbosacral fusion and to identify the risk factors associated with this complication. BACKGROUND: There are a number of well-described complications of instrumented lumbosacral fusion, including delayed stress fracture of the pelvis. Early sacral or pelvic stress fracture after instrumented lumbosacral fusion has not been previously reported, to the authors' knowledge. METHODS: The authors present three cases of early stress fracture occurring at 2-4 weeks after surgery in patients who underwent instrumented multilevel lumbosacral fusions for degenerative lumbosacral disease. RESULTS: Two patients had sacral fracture, which to the authors' knowledge, has not been previously reported. Risk factors included lumbosacral instrumentation and fusion, osteoporosis in elderly women, and iliac crest bone graft procurement. All patients were treated conservatively, with restricted ambulation and gradual return to activity. CONCLUSION: This complication can cause significant morbidity and a delay in the patient's return to function. A better understanding of the related biomechanical forces and preoperative risk factors may identify patients at risk and may aid in surgical planning and in expectations of postoperative recovery.     

Pedicle fracture after instrumented posterolateral lumbar fusion: a case report

STUDY DESIGN: The case report of a 60-year-old man with late onset back pain after lumbar spine fusion is presented. OBJECTIVE: To report the rare complication of bilateral pedicle stress fractures after instrumented posterolateral lumbar fusion. SUMMARY OF BACKGROUND DATA: A 56-year-old man underwent revision spinal surgery for ongoing back pain secondary to pseudoarthroses. A posterolateral L4-S1 instrumented fusion using pedicle screws was performed. Autologous bone graft was applied to the decorticated lateral masses. The internal fixation was removed 2 years later, at which time plain radiographs showed that the fusion mass was solid. At the age of 60 years, the man presented with worsening back pain. Plain radiographs and computed tomographic scans demonstrated bilateral L4 pedicle stress fractures. A bone scan indicated that these were recent in origin. METHOD: The clinical assessment was undertaken by the senior author and surgeon. Investigations included plain radiography, computer tomography, and scintographic imaging. A systematic literature review of the relevant publications was performed. RESULTS: In the reported patient, bilateral pedicle stress fractures developed 2 years after pedicle screw removal from an L4-S1 instrumented posterolateral lumbar spine fusion. This occurred at the uppermost level of the fusion mass. CONCLUSIONS: The pedicle is the weakest point in the neural arch after posterolateral fusion. Although movement continues at the level of the disc space anteriorly, the pedicle is susceptible to fracture. Pedicle fracture is a rare late complication of posterolateral lumbar spine fusion.     

Treatment of congenital spondyloptosis in an 18-month-old patient with a 10-year follow-up.

STUDY DESIGN: Case report. OBJECTIVES: To present the case of a patient with congenital spondyloptosis treated and followed over 10 years. SUMMARY OF BACKGROUND DATA: The surgical management of spondyloptosis in children is variably reported in the literature. Some authors propose that posterior fusion in situ is a safe and reliable procedure, whereas others suggest that reduction of the slipped vertebra may prevent some of the adverse sequelae of in situ fusion, which include nonunion, bending of the fusion mass, and persistent lumbosacral deformity. Many investigators advocate a combined anterior and posterior fusion using instrumentation. METHODS: At the time of the first symptoms an 18-month-old boy with congenital spondyloptosis of L5-S1 was referred to the authors' institution. Because of the progression of pain, neurologic disturbance, mild foot deformity, muscle contractures, and lumbosacral kyphosis, surgical intervention was undertaken. Operative intervention began with a resection of the L5 lamina and wide bilateral L5 nerve root decompression. This was followed by anterior subtotal resection of L5 and interbody bone graft of the morcelized vertebral body for fusion from L5 to S1. The next step was reduction of the spondyloptosis and stabilization by posterior instrumentation L2-S1 with a sacral Cotrel-agraffe device. RESULTS: The procedure achieved almost complete reduction of the spondyloptosis with near-normal restoration of lumbar lordosis allowing more physiologic lumbar spinal biomechanics. There were no neurologic complications. After surgery there was no suggestion of back pain or gait disturbance and no progression of any deformity. CONCLUSION: In the treatment of severe congenital spondylolisthesis a staged procedure of decompression, reduction, and instrumented fusion is recommended for those cases in which intervention is indicated.     

A novel approach to sagittal balance restoration following iatrogenic sacral fracture and resulting sacral kyphotic deformity. Technical note

The authors describe the use of sacral pedicle subtraction osteotomy (PSO) with multiple sacral alar osteotomies for the correction of sacral kyphosis and pelvic incidence and for achieving sagittal balance correction in cases of fixed sagittal deformity after a sacral fracture. In this paper, the authors report on a novel technique using a series of sacral osteotomies and a sacral PSO to correct a fixed sagittal deformity in a patient with a sacral fracture that had healed in a kyphotic position. The patient sustained this fracture after a previous surgery for multilevel instrumented fusion. Preoperative and postoperative radiographic studies are reviewed and the clinical course and outcome are presented. Experts agree that the pelvic incidence is a fixed parameter that dictates the morphological characteristics of the pelvis and affects spinopelvic orientation and sagittal spinal alignment. An increased pelvic incidence is associated with a higher degree of spondylolisthesis in the lumbosacral junction, and increased shear forces across this junction. The authors demonstrate that the pelvic incidence can be altered and corrected with a series of sacral osteotomies to improve sacral kyphosis, compensatory lumbar hyperlordosis, and sagittal balance.     

Jackson's intrasacral fixation in the management of high-grade isthmic spondylolisthesis

The aim of this study is to describe a surgical procedure proposed for high-grade isthmic spondylolisthesis, with intraoperative reduction using Jackson's intrasacral fixation. The procedure is performed using a single posterior approach. The intraoperative correction of the deformity is obtained by sacral dome resection and reduction of the lumbosacral kyphosis using Jackson's fixation to rotate the sacrum. After sagittal balance restoration, L4-S1 circumferential fusion is performed with interbody cages. The technique is effective in the restoration of spinal and pelvic parameters of sagittal balance, and optimal conditions for fusion are obtained. The use of Jackson's intrasacral fixation provides the strong stability needed to correct the lumbosacral deformity, with little neurological risk during intraoperative reduction.     

An unusual natural history of a L5–S1 spondylolisthesis presenting with a sacral insufficiency fracture

Sacral insufficiency fractures have been described in association with conditions leading to osteoporosis. No association with spondylolisthesis has been described to date. A 60-year-old patient with known lumbosacral isthmic spondylolisthesis presented with exacerbation of symptoms initially thought to be linked to her known spinal pathology. Plain radiography, computer tomography, MRI and bone scan confirmed the presence of a recent sacral insufficiency fracture with anterior angulation. Conservative treatment resulted in improvement of symptoms after 6 months. Care should be taken when considering older patients for more aggressive treatment if they present with exacerbation of back pain and sciatica in the presence of a pre-existing spondylolisthesis. A suspicion of insufficiency fracture should be raised if risk factors exist and further investigations ordered in particular if plain radiography is normal. Lumbosacral fusion might be inappropriate in this setting.     

Spondylolysis and spondylolisthesis in children and adolescents: II. Surgical management

Surgical management is indicated for children and adolescents with spondylolysis and low-grade spondylolisthesis (< or =50% slip) who fail to respond to nonsurgical measures. In situ posterolateral L5 to S1 fusion is the best option for those with a low-grade slip secondary to L5 pars defects or dysplastic spondylolisthesis at the lumbosacral junction. Pars repair is reserved for patients with symptomatic spondylolysis and low-grade, mobile spondylolisthesis with pars defects cephalad to L5 and for those with multiple-level defects. Screw repair of the pars defect, wiring transverse process to spinous process, and pedicle screw-laminar hook fixation are surgical options. The ideal surgical management of high-grade spondylolisthesis (>50% slip) is controversial. Spinal fusion has been indicated for children and adolescents with high-grade spondylolisthesis regardless of symptoms. In situ L4 to S1 fusion with cast immobilization is safe and effective for alleviating back pain and neurologic symptoms. Instrumented reduction and fusion techniques permit improved correction of sagittal spinal imbalance and more rapid rehabilitation but are associated with a higher risk of iatrogenic nerve root injuries than in situ techniques. Wide decompression of nerve roots combined with instrumented partial reduction may diminish the risk of neurologic complications. Pseudarthrosis and neurologic injury presenting as L5 radiculopathy and sacral root dysfunction are the most common complications associated with surgical management of high-grade spondylolisthesis.     

Sacral fracture as a rare early complication of lumbosacral spondylodesis

A 57 year old patient with secondary cortisone induced osteoporosis war surgically treated by means of posterior lumbar interbody fusion with internal fixation from L4-S1 for symptomatic instability in the level L4/5 after previous dorsal stabilisation L5/S1. After an unapparent initial postoperative phase, the patient complained of severe pain in the low back and gluteal region 9 day after surgery. Radiographs as well as CT-scans showed a horizontal fracture of the sacrum. After a short period of immobilisation the patient was carefully remobilised with an orthesis and the pain gradually subsided. Conventional radiographs one year later showed complete consolidation of the fracture and a good clinical result. There are only two literature reports with together 3 cases of patients attaining an early sacral fracture after spondylodesis. The main cause seems to be the unphysiological biomechanical stress placed on the osteoporotic sacrum after moresegmental spondylodesis. Further risk factors seem to be adipositas, female gender and age. Despite the rarity of sacral fractures after lumbosacral fusion, this complication should at least be considered in the differential diagnosis in patients who complain of persisting or sudden-onset pain after surgery.     

Surgical treatment of sacral fractures following lumbosacral arthrodesis: Case report and literature review

Sacral fractures following posterior lumbosacral fusion are an uncommon complication. Only a few case series and case reports have been published so far. This article presents a case of totally displaced sacral fracture following posterior L4-S1 fusion in a 65-yearold patient with a 15-year history of corticosteroid use who underwent open reduction and internal fixation using iliac screws. The patient was followed for 2 years. A thorough review of the literature was conducted using the Medline database between 1994 and 2014. Immediately after the revision surgery, the patient's pain in the buttock and left leg resolved significantly. The patient was followed for 2 years. The weakness in the left lower extremity improved gradually from 3/5 to 5/5. In conclusion, the incidence of postoperative sacral fractures could have been underestimated, because most of these fractures are not visible on a plain radiograph. Computed tomography has been proved to be able to detect most such fractures and should probably be performed routinely when patients complain of renewed buttock pain within 3 mo after lumbosacral fusion. The majority of the patients responded well to conservative treatments, and extending the fusion construct to the iliac wings using iliac screws may be needed when there is concurrent fracture displacement, sagittal imbalance, neurologic symptoms, or painful nonunion.     

腰骶内固定的适应证与主要技术

腰骶融合手术为脊柱外科中常用术式,而腰骶内固定有助于腰骶融合。其手术适应证包括腰椎滑脱椎间盘突出与椎管狭窄、畸形以及其他原因缃起的腰骶不稳。腰骶内固定的主要技术有Ｇａｌｖｅｓｔｏｎ技术、髂骨螺钉、骶骨椎弓根螺钉、骶骨构、骶骨棒、骶骨杆、关节突螺钉、腰骶椎体间融合、人工椎间盘等。     

Iatrogenic spondylolysis leading to contralateral pedicular stress fracture and unstable spondylolisthesis: a case report

STUDY DESIGN: A case report of iatrogenic spondylolysis as a complication of microdiscectomy leading to contralateral pedicular stress fracture and unstable spondylolisthesis. OBJECTIVE: To improve understanding of this condition by presenting a case history and roentgenographic findings of a patient that differ from those already reported and to propose an effective method of surgical management. METHODS: A 67-year-old woman with no history of spondylolysis or spondylolisthesis underwent an L4-L5 microdiscectomy for a left herniated nucleus pulposus 1 year before the current consultation. For the preceding 8 months, she had been experiencing low back and bilateral leg pain. Imaging studies revealed a left L4 spondylolytic defect and a right L4 pedicular stress fracture with an unstable Grade I spondylolisthesis. RESULTS: The patient was treated with posterior spinal fusion, which resulted in complete resolution of her clinical and neurologic symptoms. CONCLUSIONS: Iatrogenic spondylolysis after microdiscectomy is an uncommon entity. However, it can lead to contralateral pedicular stress fracture and spondylolisthesis, and thus can be a source of persistent back pain after disc surgery. Surgeons caring for these patients should be aware of this potential complication.     

Incidence,diagnosis, and management of sacral fractures following multilevel spinal arthrodesis

BackgroundFractures of the sacrum are a rare complication following instrumented spinal fusion, with only 34 cases previously reported in the literature. Previous series have generally been limited to less than five cases.PurposeThe purpose of this study is to determine the incidence of sacral fractures caudal to instrumented spinal fusion constructs, identify risk factors for fracture and for failure of conservative management, and describe strategies for surgical treatment of these fractures.Study designThis is a retrospective review.Patient samplePatients undergoing instrumented posterior spinal arthrodesis between 2002 and 2011 were included in the sample.Outcome measuresClinical and radiographic data from hospital and surgeon records comprise outcome measures.MethodsMethods include a review of clinical and radiographic data from a prospectively collected patient database recording all adjacent segment fractures during the study period.ResultsTwenty-four patients developed sacral fractures caudal to instrumented spinal fusion constructs during the study period. The overall incidence was 6.1% and was significantly greater in fusions greater than four levels (14.5%). The mean time from index surgery to fracture was 4.3 months. Only one fracture was evident on plain radiography at the onset of symptoms. Computed tomography, magnetic resonance imaging, and nuclear scintigraphy can all be used to establish the diagnosis. Eight patients were successfully treated conservatively. The mean time to fracture union was 21 weeks. Anterolisthesis of the fracture greater than 2 mm and kyphotic angulation were significantly associated with failure of conservative management. Surgical intervention included posterior extension of the fusion construct to S2 and the iliac wings with sacroiliac joint fusion. In 10 cases, a combined anterior and posterior approach was used that consisted of either revision anterior lumbar interbody fusion or transsacral posterior lumbar interbody fusion.ConclusionsSacral fractures following instrumented posterior spinal fusion are an uncommon complication; that is often unrecognized on plain radiographs. Risk factors include osteoporosis and long spinal fusions. Anterolisthesis and kyphosis of the fracture is associated with failure of conservative management.     

Surgery for severe spondylolisthesis and spondyloptosis

Objective  Restoration of a normal profile of spinal column by complete or almost complete reduction, stabilization achieved by instrumentation and fusion of the lumbosacral intervertebral segment. Alleviation or at least marked reduction of pain and neurologic deficits present before surgery. Indications  High grade, that means usually spondylolisthesis grade IV according to Meyerding or spondyloptosis. Patients with progression of slip. Contraindications  Osteopenia. Spondylolistheses which do not necessitate an opening of the spinal canal. Possibility of adequate anterior access to the lumbosacral disk. Surgical Technique  Posterior decompression and resection of the sacral dome, reduction of L5 over the sacrum and posterior interbody fusion L5/S1 with two autogenous bi- or tricortical bone grafts harvested from the iliac crest. Results  Between January 1995 and January 1998, we used the described technique in eleven patients. Six patients had a grade IV spondylolisthesis and five a spondyloptosis. Previous surgery had been done in four patients. A complete or almost complete reduction was possible in ten patients. No pseudarthrosis nor loss of correction were seen. Neurologic deficits improved in five of six patients. All patients noted an improvement of symptoms. Postoperative complications in previously operated patients: one dura lesion, one deficit of the S1 nerve root diagnosed postoperatively, and one inadequate reduction. Postoperative complications in the remaining seven, not previously operated patients: one instability of the adjacent segment and a transient irritation of the L5 nerve root in two patients.     

Knodt rod distraction instrumentation in lumbosacral arthrodesis

Review of 40 patients undergoing lumbosacral fusions over a 4-year period was done to determine the value, efficiency, and safety of Knodt rod distraction instrumentation. The age range was 30-80 years. Mean age was 51 years. Follow-up was 1-4 years. Twenty patients underwent decompression and fusion for spinal stenosis, nine underwent spinal arthrodesis for instability, six underwent the same for spondylolisthesis, and five underwent fusions for other diagnoses. A posterior midline approach was used. Laminal hook sites were prepared, and care was taken to prevent dural compression or tenting. Balanced distraction was done to restore soft tissue tension and stability. No attempt was made to reduce deformity. A posterior and lateral mass fusion augmented with allograft bone was performed on all but three patients, in whom autogenous bone was used. The majority of patients were placed in a custom-molded lumbosacral orthosis for 3-6 months after operation. There were no neurologic complications, dural tears, or pseudomeningoceles. The first sacral laminas were instrumented in 22 patients. Nine of the 40 patients underwent rod removal. Reasons for removal were pain due to loosening in five patients and failure of fusion in two. On rod removal in two patients, no abnormality was found. Insertion within the sacral laminas did not lead to neurologic complications. The major problem appeared to be loosening, which necessitated rod removal in 12% of the patients. Knodt rod distraction instrumentation is a safe and effective method of internal fixation for lumbosacral fusions.     

Anterior lumbar interbody fusion for lumbosacral junction in steep sacral slope

STUDY DESIGN: Retrospective study of surgical technique and clinical outcome. OBJECTIVES: To examine the technique and outcomes of anterior lumbar interbody fusion (ALIF) surgery for a lumbosacral junction in a steep sacral slope. SUMMARY OF BACKGROUND DATA: There are no studies on the outcome and technical pitfalls on ALIF surgery for a lumbosacral junction in a steep sacral slope. MATERIALS AND METHODS: Six female patients (mean age of 55.67 y; range, 42 to 69) who had a steep sacral slope underwent ALIF surgery for degenerative (2 patients) and spondylolytic (4 patients) spondylolisthesis. The average follow-up duration was 29.33 months (range, 27 to 33 mo). The following parameters were used to assess the outcomes: slip angle, slip percentage, sacral inclination, lumbosacral angle, sacral slope, and distance from the upper margin of the pubis symphysis to the sacral plane. The level of pain was measured using the visual analog pain scale score. The function was assessed using the Oswestry Disability Index (ODI) score. Satisfaction surveys were also carried out. Statistical analysis was performed using a Friedman test. A P value <0.05 was considered significant. RESULTS: The mean sacral inclination, lumbosacral angle, sacral slope, and distance from the upper margin of the pubis symphysis to the sacral plane were 37.34 degrees (range, 28.55 to 48.92 degrees), 12.20 degrees (range, 5.09 to 16.5 degrees), 40.70 degrees (range, 30.54 to 49.98 degrees), and 22.06 cm (range, 16.13 to 29.72 cm), respectively. The mean correction of slip percentage and slip angle was 35.46%, and 9.3 degrees, respectively. The mean visual analog pain scale score decreased from 8.5 (back pain) and 7.3 (leg pain) to 1.8 (back pain) and 1.8 (leg pain) after surgery (P=0.001). The mean ODI scores also reflected the improved status (ODI of 64.7 before surgery to 8.5 after surgery; P=0.001). The patient's satisfaction was relatively high. All the patients had radiographically solid fusion at the latest follow-up. There were no significant complications encountered in this study. CONCLUSIONS: In selected cases, a steep sacral slope may not be an absolute contraindication of ALIF. Moreover, the C-arm-guided reduction and cage insertion method is a reliable way of treating spondylolisthesis in those with a steep sacral slope.     

Retained surgical sponges, a denied neurosurgical reality? Cautionary note

Surgically acquired foreign bodies are well known but not widely reported. Only seven articles pertaining to this subject were found in the current neurosurgical literature. Are they a denied neurosurgical reality? In this report with a concededly provoking title, the authors elucidate clinical and medicolegal aspects of retained surgical sponges, with emphasis on spinal procedures. To highlight particulars, a case is presented in which a retained surgical sponge was encountered as the cause of progressive low back pain and tender swelling in the scar area after instrumented posterolateral lumbar spinal fusion combined with pedicle screw fixation for lumbosacral spondylolisthesis 4 years earlier. However, until today, no reported neurosurgical patient has suffered a serious complication due to a retained surgical sponge. The authors wish to remind the neurosurgical community to learn from unpleasant clinical and medicolegal experiences in other specialties before serious complications occur, and we suggest rigorous standardization of intraoperative habits to avoid this hazardous complication.     

Facet fusion in the lumbosacral spine: a 2-year follow-up study

OBJECTIVE: A clinical and radiological follow-up study was undertaken to assess the safety, efficacy, and complication rate associated with instrumented facet fusion of the lumbar and lumbosacral spine. METHODS: This study involved 99 patients with degenerative lumbar disorders who were treated surgically at the authors' neurosurgical department and followed for more than 2 years. Eighty-two patients underwent one-level fusion for the treatment of Grade I or II degenerative spondylolisthesis and accompanying spinal canal stenosis (44 patients) or recurrent disc herniation (38 patients). Seventeen patients underwent two-level fusion for the treatment of either double instances of the above indications (seven patients) or concurrent stenosis at the adjacent level (10 patients). RESULTS: There were no technique-related complications. The overall 2-year success rate of fusion was 96%; the success rates by fusion type were 99% in one-level fusions and 88% in two-level fusions. Degenerative spondylolisthesis had the highest success rate at 100%, whereas the success rate in patients who had not responded to previous discectomy was 93%. Patients with concurrent stenosis experienced the lowest success rate: 80%. Excellent or good clinical results were obtained for 85% of patients with one-level fusions and for 65% of patients with two-level fusions. CONCLUSION: Instrumented facet fusion alone is a simple, safe, and effective surgical option for the treatment of patients with single-level disorders, especially patients with degenerative spondylolisthesis.     

Anterior fibular strut grafting for the treatment of pseudoarthrosis in spondylolisthesis

Ten patients with a failed posterior spinal fusion for symptomatic spondylolisthesis were treated with retroperitoneal anterior lumbosacral interbody fusion. A fibular strut allograft was placed, followed by posterolateral fusion and instrumentation. The mean follow-up was 40 months (range 24-60 months). All patients complained of back pain and leg pain before surgery. All patients achieved solid fusion at L5-S1. One patient developed pseudoarthrosis at L4-5 and improved symptomatically with no postoperative complications.     

Transverse fracture-dislocation of the sacrum: a diagnostic pitfall and a surgical challenge

Transverse fracture-dislocations of the sacrum are rare. Associated lesions of the lumbosacral spine as well as neurological injuries are common. Conventional radiographs of the pelvis often fail to clearly visualize the fracture. Delayed diagnosis increases the risk of progressive neurological disfunction. True lateral sacral views and CT-scans with 3-dimensional reconstructions are very helpful in establishing the full extent of the injury. These examinations should be considered in all patients with a history of high energy trauma and clinical signs indicating lumbosacral injury, such as severe low back pain and neurological disturbances of the lower extremities. The management of transverse sacral fracture-dislocations with or without associated neurological damage is controversial. Conservative treatment is associated with a high rate of persistent deformity and residual neurological dysfunction. Surgical management allows for anatomical fracture reduction, stable fixation and revision of the spinal canal and lumbosacral nerve roots. The dorsal approach is preferred. Two patients with transverse sacral fracture-dislocations and neurological disturbances are presented. One patient had an additional fracture-dislocation of the lumbar spine at the L4L5 level with intrusion of the lumbosacral spine into the pelvis. Both lesions in this patient were successfully stabilized using an internal fixator system. The other patient presented with a bilateral transforaminal sacral fracture. The transverse component was not recognized on the initial radiographs, which resulted in loss of reduction and progressive neurological disfunction after sacroiliac screw fixation.     

Augmentation of a loosened sacral pedicle screw with percutaneous polymethylmethacrylate injection

STUDY DESIGN: Case report describing augmentation of a loosened sacral screw with percutaneous injection of polymethylmethacrylate. OBJECTIVE: To highlight the advantages of percutaneous injection of polymethylmethacrylate in treatment of loosened pedicle screws. SETTING: Turkey. SUMMARY OF BACKGROUND DATA: Lumbosacral instrumentation with pedicle screws is a commonly used procedure for correcting deformity and stabilizing the spine until bony fusion occurs. Loosening of the screws is a complication of this procedure and is seen more frequently in multilevel instrumentation, particularly in those instrumentations using sacral screws. METHOD: We present the case of a woman who had had L3-L4-L5-S1 instrumentation with pedicle screws who developed severe low back pain radiating to the left hip 2 months after the operation. Radiologic and clinical examination revealed that the left pedicle screw of the sacrum had loosened, and revision surgery was planned. The left sacral screw was augmented with percutaneous injection of polymethylmethacrylate. RESULTS: The patient was free of pain after the operation. CONCLUSIONS: We suggest that percutaneous polymethylmethacrylate injection around a loosened screw may be an alternative to major open revision surgery in cases of sacral screw failure in multisegmental instrumentation.