Gilad J. Regev

Morphometric Analysis of the Ventral Nerve Roots and Retroperitoneal Vessels With Respect to the Minimally Invasive Lateral Approach in Normal and Deformed Spines

Study Design. A morphometric analysis, using magnetic resonance imaging (MRI) studies of the lumbar spine. Objective. To identify the anatomic position of the ventral root and the retroperitoneal vessels in relation to the vertebral body in normally aligned and deformed spines. Summary of Background Data. The lateral approach to the lumbar spine is a relatively new method for performing interbody fusions. In contrast to the standard open anterior approach with direct vision of the operative field, the lateral approach uses expandable retractors that are positioned under fluoroscopic guidance. Risks of this technique include injury to the exiting nerve root and retroperitoneal vessels. Methods. One hundred lumbar spine MRI studies were reviewed from patients treated for various spinal pathologies. The measured intervertebral segments were divided into 3 groups: group 1 (n = 247), normally aligned vertebrae and disc spaces; group 2 (n = 18), degenerative spondylolisthetic segments; and group 3 (n = 19), segments from the apex of degenerative lumbar scoliosis. Axial MR images were used to measure: the vertebral endplate anterior-posterior (AP) diameter, the overlap between the ventral root and the posterior margin of the vertebra, and the overlap between the retroperitoneal large vessels and the anterior edge of the vertebra. Results. The overlap between the adjacent neuro-vascular structures and the vertebral body endplate gradually increased from L1–L2 to L4–L5. The maximal overlap, at the L4–L5 level reached 87% resulting in a relatively narrow corridor for performing the operative procedure. Alteration in the anatomic location of the nerve root and the retroperitoneal vessels, in Group 3 (scoliosis) further decreased the safe corridor. Conclusion. The safe corridor for performing the discectomy and inserting the intervertebral cage narrows from L1–L2 to the L4–L5 level. This corridor is further narrowed with rotatory deformity of the spine. Using the preoperative MRI to assess the relative position of the adjacent neuro-vascular structures in relation to the lower vertebra’s endplate at each level is recommended.

Nerve injury to the posterior rami medial branch during the insertion of pedicle screws: comparison of mini-open versus percutaneous pedicle screw insertion techniques.

Study Design. The risk for transection of the medial branch nerve (MBN) after minimally invasive insertion of pedicle screws was evaluated in a human cadaver model. Objective. The purpose of this study is to compare the risk of MBN transection after pedicle screw insertion using mini-open versus percutaneous minimally invasive techniques. Summary of Background Data. The multifidus muscle is innervated by the MBN that originates from the posterior rami. Since the anatomic course of the MBN passes near the mamillary process it is vulnerable to injury during pedicle screw insertion, even if minimally invasive spine techniques are implemented. Methods. Five cadaveric specimens were used for the study. Pedicle screws were inserted into the lumbar spine using either percutaneous or mini-open techniques. The integrity of the MBN was examined directly through anatomic dissection of the posterior spine. Results. The soft tissue damage area around the screw insertion site was significantly greater in the mini-open compared with the percutaneous technique. MBN transection was observed in 84% (21/25) of the pedicles when using the mini-open technique and in 20% (5/25) when the percutaneous insertion technique was used (P < 0.01%). Conclusion. Using a percutaneous technique for pedicle screw insertion significantly reduces the risk of injury to the MBN. We therefore recommend using this technique especially at the most cephalic levels to minimize the risk of denervating the multifidus muscle fascicles that originate from the adjacent mobile level.

Psoas muscle architectural design, in vivo sarcomere length range, and passive tensile properties support its role as a lumbar spine stabilizer.

Study Design. Controlled laboratory and cross-sectional study designs. Objective. To determine psoas major (PM) muscle architectural properties, in vivo sarcomere-length operating range, and passive mechanical properties. Summary of Background Data. PM is an important hip flexor but its role in lumbar spine function is not fully understood. Several investigators have detailed the gross anatomy of PM, but comprehensive architectural data and in vivo length-tension and passive mechanical behaviors have not been documented. Methods. PM was isolated in 13 cadaver specimens, permitting architectural measurements of physiological cross-sectional area (PCSA), normalized fiber length (Lf), and Lf:muscle length (Lm) ratio. Sarcomere lengths were measured in vivo from intraoperative biopsies taken with the hip joint in flexed and extended positions. Single-fiber and fiber bundle tensile properties and titin molecular weight were then measured from separate biopsies. Results. Architecturally, average PCSA was 18.45 ± 1.32 cm2, average Lf was 12.70 ± 2 cm, and average Lf: Lm was 0.48 ± 0.06. Intraoperative sarcomere length measurements revealed that the muscle operates from 3.18 ± 0.20 &mgr;m with hip flexed at 10.7° ± 13.9° to 3.03 ± 0.22 &mgr;m with hip flexed at 55.9° ± 21.4°. Passive mechanical data demonstrated that the elastic modulus of the PM muscle fibers was 37.44 ± 9.11 kPa and of fiber bundles was 55.3 ± 11.8 kPa. Conclusion. Analysis of PM architecture demonstrates that its average Lf and passive biomechanical properties resemble those of the lumbar erector spinae muscles. In addition, PM sarcomere lengths were confined to the descending portion of the length-tension curve allowing the muscle to become stronger as the hip is flexed and the spine assumes a forward leaning posture. These findings suggest that the human PM has architectural and physiologic features that support its role as both a flexor of the hip and a dynamic stabilizer of the lumbar spine.

Use of a quantitative pedicle screw accuracy system to assess new technology: Initial studies on O-arm navigation and its effect on the learning curve of percutaneous pedicle screw insertion.

Background A quantitative screw accuracy system is proposed that allows for high-fidelity discrimination between various methods of pedicle screw insertion. Our purpose was to study the utility of a quantitative screw accuracy scoring system to assess new imaging technologies and their effects on the minimally invasive spine learning curve. Methods By use of a hypothetical “perfect screw,” a scoring system is proposed that may be used to compare the position of a small number of screws inserted according to a desired optimal position. This study incorporates a retrospective review of imaging studies for 10 patients who underwent percutaneous pedicle screw placement with either navigation-assisted O-arm imaging or navigation-assisted C-arm imaging. For the learning-curve portion of the study, 2 cadaveric adult torsos were used for instrumentation. Computed tomography imaging studies were used in both studies to assess screw position in the pedicle and vertebral body in relation to an optimal screw by use of a quantitative scoring system to rate accuracy. Results The quantitative scoring system allowed a statistically significant accuracy difference to be ascertained between 2 different technologies using fewer data points than previously published methods. When this screw scoring system is applied to minimally invasive percutaneous pedicle screw insertion, an optimal screw position can be achieved with greater accuracy through navigation-assisted technology (O-arm with computer-assisted navigation). When the O-arm with computer-assisted navigation was used by a novice surgeon learning the technique of percutaneous screw insertion, screws were inserted in a shorter period without loss of accuracy. In contrast, use of the traditional C-arm fluoroscopy leads to a loss of accuracy with faster insertion times. Increased accuracy can be seen clinically when compared with fluoroscopic navigation. Conclusions The use of a quantitative scoring system allows for rapid assessment of screw accuracy. As additional technologies and new teaching techniques for pedicle screw insertion are developed, this scoring system may be useful as an early assessment tool.

Regional Myosin Heavy Chain Distribution in Selected Paraspinal Muscles

Study Design. Cross-sectional study with repeated measures design. Objective. To compare the myosin heavy-chain isoform distribution within and between paraspinal muscles and to test the theory that fiber-type gradients exist as a function of paraspinal muscle depth. Summary of Background Data. There is still uncertainty regarding the fiber-type distributions within different paraspinal muscles. It has been previously proposed that deep fibers of the multifidus muscle may contain a higher ratio of type I to type II fibers, because, unlike superficial fibers, they primarily stabilize the spine, and may therefore have relatively higher endurance. Using a minimally invasive surgical approach, using tubular retractors that are placed within anatomic intermuscular planes, it was feasible to obtain biopsies from the multifidus, longissimus, iliocostalis, and psoas muscles at specific predefined depths. Methods. Under an institutional review board-approved protocol, muscle biopsies were obtained from 15 patients who underwent minimally invasive spinal surgery, using the posterior paramedian (Wiltse) approach or the minimally invasive lateral approach. Myosin heavy chain (MyHC) isoform distribution was analyzed using SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis) electrophoresis. Because multiple biopsies were obtained from each patient, MyHC distribution was compared using both within- and between-muscle repeated measures analyses. Results. The fiber-type distribution was similar among the posterior paraspinal muscles and was composed of relatively high percentage of type I (63%), compared to type IIA (19%) and type IIX (18%) fibers. In contrast, the psoas muscle was found to contain a lower percentage of type I fibers (42%) and a higher percentage of type IIA (33%) and IIX fibers (26%; P < 0.05). No significant difference was found for fiber-type distribution among 3 different depths of themultifidus and psoas muscles. Conclusion. Fiber-type distribution between the posterior paraspinal muscles is consistent and is composed of relatively high percentage of type I fibers, consistent with a postural function. The psoas muscle, on the other hand, is composed of a higher percentage of type II fibers such as in the appendicular muscles. Our data do not support the idea of a fiber-type gradient as a function of depth for any muscle studied.

Evaluation of hip flexion strength following lateral lumbar interbody fusion

BACKGROUND CONTEXT Lateral interbody fusion (LIF) is a minimally invasive procedure that is designed to achieve a solid interbody fusion while minimizing the damage to the surrounding soft tissue. Although short-term results have been promising, few data have been published to date regarding its risks and complication rate. PURPOSE The aim was to evaluate the extent of injury to the psoas muscle after the LIF procedure by measuring hip flexion strength. STUDY DESIGN A prospective case series was used in the study. METHOD Hip flexion strength was measured using a handheld digital dynamometer while the patient was seated on a chair; the examiner held the device against the patients attempt to flex the hip. Both sides were measured to compare the operated and nonoperated psoas muscles. Each side was measured three times and the average amount (in pounds) was recorded. Measurements were done before and after surgery on Day 2-3, at 2 weeks, 6 weeks, and at 3 and 6 months. RESULTS Thirty-three patients were recruited for this study. Mean preoperative hip flexion strength values were 20.7±3.47 lb and 21.3±4.31 lb for operated and nonoperated legs, respectively, with no significant difference (p=.85). With a mean of 11.2±2.24 lb postoperative measurements on Day 2, the operated side showed statistically significant reduction of strength (p=.0001). The nonoperated side was also weaker postoperatively, but not significantly (mean=19.12±1.74 lb; p=.097). From the first follow-up visit at 2 weeks, the values on the operated leg had returned to baseline values (20.6, p=.97) and were not significantly different from preoperative values on either side. DISCUSSION Hip flexion was weakened immediately after the LIF procedure, which may be attributed to psoas muscle injury during the procedure. However, this damage was temporary, with almost complete return to baseline values by 2 weeks.

Admission Norton scale scores (ANSS) are associated with post-operative complications following spine fracture surgery in the elderly

We sought to determine if low ANSS, usually associated with high pressure ulcer risk, are also associated with post-operative complications following spine fracture surgery in the elderly. This was a retrospective cross-sectional study conducted at the division of orthopedic surgery in a tertiary medical center between January 2008 and October 2010. The medical charts of consecutive elderly (≥ 65 years) patients admitted for spine fracture surgery were studied for the following measurements: ANSS, demographic data, co-morbidities, and post-operative complications. Except for pressure ulcers, post-operative complications included: acute coronary syndrome, acute renal failure, confusion, pneumonia, urinary tract infection, venous thromboembolism, and wound infection. The final cohort included 90 patients: 66 (73.3%) females and 24 (26.7%) males. Mean age for the entire cohort was 78.9 ± 0.7 years. Most patients had lumbar fractures (n=49; 54.4%) or thoracal fractures (n=26; 28.9%). Most patients underwent kyphoplasty (n=65; 72.2%). Mean ANSS was 15.9 ± 0.3, and 29 (32.2%) patients had low (<15) ANSS. Patients with low ANSS had significantly more post-operative complications relative to patients with high ANSS (1.0 ± 0.2 vs. 0.2 ± 0.1; p<0.0001). Among all post-operative complications, urinary tract infection was independently associated with ANSS (p<0.0001). Binary regression analysis showed that ANSS were independently associated with post-operative complications (p=0.001). We conclude that low ANSS are associated with post-operative complications and urinary tract infection in particular, following spine fracture surgery in the elderly. Hence, the Norton scoring system may be used for predicting and preventing post-operative complications in this population.

Incidence and prevention of intervertebral cage overhang with minimally invasive lateral approach fusions.

Study Design. Radiographic review. Objective. To evaluate the incidence and degree of cage overhang in minimally invasive spinal (MIS) fusions, when using either the direct lateral interbody fusion (DLIF) or extreme lateral interbody fusion (XLIF) techniques. Summary of Background Data. Among the difficulties surgeons face during a MIS lateral interbody fusion is to assess the proper placement of the cage without the use of direct visualization. Determining the proper length of the cage using AP view fluoroscopy can be misleading. As the axial profile of the vertebral body is oval, inserting the cage anterior or posterior to the maximal width point requires adjustment of the cages length. Methods. The incidence and degree of cage overhang were measured using magnetic resonance imaging (MRI) and computed tomography (CT) studies from patients that underwent a MIS lateral interbody fusion. To determine the adjustment needed when the cage is inserted at various sagittal sites, the coronal spans of normal vertebral endplates were measured. Results. Forty-five percent of the cages were placed in the central portion, 34% were located in the anterior 1/3, and 7% were located in the posterior 1/3 of the disc space. Of the anterior positioned cages, 45% were found to be overhanging outside of the boundaries of the intervertebral disc space. The average measured lateral protrusion was 7.8 ± 3.6 mm, and anterior protrusion was 9.8 ± 3.3 mm. The vertebral body width measured 41.7 ± 6 mm at the anterior 1/3, 50 ± 4 mm at the mid, and 49 ± 1 mm at the posterior 1/3. Compared with the midvertebral width, the vertebral body width at the anterior 1/3 was decreased by 16.5% ± 0.9% (P < 0.05). Conclusion. The risk of placing an excessively long cage, when the insertion site is located in the anterior 1/3 of the disc, is relatively high, when performing MIS lateral approach interbody fusions. When using an anterior entry point for the insertion of the cage, choosing a 15% shorter cage length compared with that measured on the AP should prevent anterolateral protrusion of the cage.

Absorbable anterior cervical plate for corpectomy and fusion in a 2-year-old child with neurofibromatosis

Postlaminectomy cervical kyphosis is one of the most challenging entities in spine surgery. Correction of this deformity usually requires anterior fusion with plating and a strut graft or interbody cage and posterior fusion with screws and rods. The situation is more complicated in the young child because fusion may affect future growth of the cervical spine. There is also a paucity of adequate instrumentation for the small bony structures. Some authors have reported utilization of absorbable cervical plates for fusion in pediatric patients with favorable results. The authors present a modified surgical technique that was used for circumferential fusion in a 2-year-old girl with cervical kyphosis and recurrent neurofibroma. Anterior fusion was performed using an autologous rib graft and an absorbable cervical plate. This was followed by posterior fusion using rib bone and cables. Previous reports on the use of absorbable cervical plates are reviewed and the advantages of the current technique are discussed.

A retrospective review of long anterior fusions to the sacrum

BACKGROUND CONTEXT In the setting of tumor, infection, or trauma, a corpectomy of the L5 vertebral body may be necessary. However, the space has an irregular trapezoidal shape, and the failure to account for this may lead to improper fitting of the titanium cages or the allograft struts when performing a reconstruction. PURPOSE The purpose of this study was to evaluate the failure rate of implants used to reconstruct the anterior lumbar spine when an L5 corpectomy has been performed. METHODS A retrospective review of the medical records and radiographs of 19 consecutive patients undergoing an L5 corpectomy and anterior spinal fusion was performed. The radiographs were reviewed for implant failure and successful fusions. RESULTS Cases included osteomyelitis (13), fractures (4), and tumor (2). Anterior reconstruction was performed with a straight cylindrical titanium cage in six cases, allograft in six cases, iliac crest bone graft (ICBG) in two cases, and cages with lordosis built into the cage or end plates in five cases. In the six straight cylindrical titanium cages, four cases had displaced anteriorly, necessitating revision surgery. In the other two cases, both had poor fixation to the sacrum and developed nonunions. In the six reconstructed with allograft, all three fibular struts developed nonunions. In the three reconstructed with humeral or femoral allograft, all patients formed a solid fusion. In the patients reconstructed with ICBG, one formed a nonunion, whereas the other one formed a solid fusion. In the cages with lordosis built into the cage or end plates, all five developed solid fusions. CONCLUSIONS A corpectomy of L5 resulting in an irregular trapezoidal shape must be accounted for when performing the reconstruction. Use of straight cylindrical cages or allograft with small footprints may lead to an increased rate of failure. When performing the reconstruction, adding approximately 20° to 30° of lordosis to the construct may create a better fit and increase stability and result in an improved fusion rate. If using allograft, using a larger graft with greater end plate contact may also improve fusion rates.