Choll W. Kim

Architectural Analysis and Intraoperative Measurements Demonstrate the Unique Design of the Multifidus Muscle for Lumbar Spine Stability

BACKGROUND Muscular instability is an important risk factor for lumbar spine injury and chronic low-back pain. Although the lumbar multifidus muscle is considered an important paraspinal muscle, its design features are not completely understood. The purpose of the present study was to determine the architectural properties, in vivo sarcomere length operating range, and passive mechanical properties of the human multifidus muscle. We hypothesized that its architecture would be characterized by short fibers and a large physiological cross-sectional area and that it would operate over a relatively wide range of sarcomere lengths but would have very stiff passive material properties. METHODS The lumbar spines of eight cadaver specimens were excised en bloc from T12 to the sacrum. Multifidus muscles were isolated from each vertebral level, permitting the architectural measurements of mass, sarcomere length, normalized fiber length, physiological cross-sectional area, and fiber length-to-muscle length ratio. To determine the sarcomere length operating range of the muscle, sarcomere lengths were measured from intraoperative biopsy specimens that were obtained with the spine in the flexed and extended positions. The material properties of single muscle fibers were obtained from passive stress-strain tests of excised biopsy specimens. RESULTS The average muscle mass (and standard error) was 146 +/- 8.7 g, and the average sarcomere length was 2.27 +/- 0.06 microm, yielding an average normalized fiber length of 5.66 +/- 0.65 cm, an average physiological cross-sectional area of 23.9 +/- 3.0 cm(2), and an average fiber length-to-muscle length ratio of 0.21 +/- 0.03. Intraoperative sarcomere length measurements revealed that the muscle operates from 1.98 +/- 0.15 microm in extension to 2.70 +/- 0.11 microm in flexion. Passive mechanical data suggested that the material properties of the muscle are comparable with those of muscles of the arm or leg. CONCLUSIONS The architectural design (a high cross-sectional area and a low fiber length-to-muscle length ratio) demonstrates that the multifidus muscle is uniquely designed as a stabilizer to produce large forces. Furthermore, multifidus sarcomeres are positioned on the ascending portion of the length-tension curve, allowing the muscle to become stronger as the spine assumes a forward-leaning posture.

Use of navigation-assisted fluoroscopy to decrease radiation exposure during minimally invasive spine surgery

BACKGROUND Minimally invasive surgery decreases postoperative pain and disability. However, limited views of the surgical field require extensive use of intraoperative fluoroscopy that may expose the surgical team to higher levels of ionizing radiation. PURPOSE To assess the feasibility and safety of navigation-assisted fluoroscopy during minimally invasive spine surgery. STUDY DESIGN A combined cadaveric and human study comparing minimally invasive transforaminal lumbar interbody fusion (MIS TLIF) using navigation-assisted fluoroscopy with standard intraoperative fluoroscopy to determine differences in surgical times and radiation exposures. METHODS Eighteen fresh cadaveric spines underwent unilateral MIS TLIF by using either navigation-assisted fluoroscopy or standard fluoroscopy. Times for specific surgical steps were compared. In addition, a prospective short-term evaluation of the intraoperative and perioperative results of 10 patients undergoing navigation-assisted MIS TLIF (NAV group) compared with a retrospective review of 8 patients undergoing MIS TLIF performed by using standard fluoroscopy (FLUORO group). RESULTS In the cadaveric study, the times were similar between the NAV group and the FLUORO group for most key steps. No statistically significant differences were obtained for approach, exposure, screw insertion, facetectomy/decompression, or total surgical times. Statistically significant differences were obtained for the setup time and total fluoroscopy time. The setup time for the NAV group averaged 9.67 (standard deviation [SD], 3.74) minutes compared with 4.78 (SD, 2.11) minutes for the FLUORO group (p=.034). The total fluoroscopy time was higher for the FLUORO group compared with the NAV group (41.9 seconds vs. 28.7 seconds, p=.042). Radiation exposure was undetectable when navigation-assisted fluoroscopy is used (NAV group). In contrast, an average 12.4 milli-REM (mREM) of radiation exposure is delivered to the surgeon during unilateral MIS TLIF procedure without navigation (FLUORO group). In the clinical series, the total fluoro time for the NAV group was 57.1 seconds (SD, 37.3; range, 18-120) compared with 147.2 seconds (SD, 73.3; range, 73-295) for FLUORO group (p=.02). No statistically significant differences are noted for operating time, estimated blood loss, or hospital stay. No inadvertent durotomies, postoperative weakness, or new radiculopathy were noted in the NAV group. One inadvertent durotomy was encountered in the FLUORO group that was repaired intraoperatively without clinical sequelae. CONCLUSION The use of navigation-assisted fluoroscopy is feasible and safe for minimally invasive spine surgery. Radiation exposure is decreased to the patient as well as the surgical team.

Short-segment fixation of lumbar burst fractures using pedicle fixation at the level of the fracture.

Study Design. Cadaveric biomechanical study and retrospective chart review. Objective. Biomechanical comparison of segmental versus nonsegmental fixation of lumbar burst fractures and clinical analysis of short-term radiographic outcomes. Summary of Background Data. Traditional short nonsegmental posterior fixation of thoracolumbar burst fractures suffers from high rates of failure. Construct stability may be improved by inserting additional screws at the fracture level. Methods. Six intact human spines (L1–L3) were biomechanically tested in flexion-extension, lateral bending, and axial torsion. The inferior half of the L2 vertebral bodies and L2–L3 discs were resected to mimic an unstable L2 burst fracture with loss of anterior column support. Pedicle screws were inserted in L1 and L3 for the control group (nonsegmental fixation). Screws were inserted at all levels for the experimental group (segmental fixation). Construct stiffness and L1–L2 disc pressure were analyzed. Twelve patients with thoracolumbar burst fractures treated with this type of segmental fixation were reviewed. Results. Construct stiffness during axial torsion was significantly higher for segmental constructs compared with nonsegmental constructs (P < 0.02) with no differences between flexion-extension and lateral bending. Disc pressure fluctuations during flexion-extension were significantly higher for segmental compared with nonsegmental constructs (P < 0.02), with no differences in lateral bending and torsion. Mean preoperative kyphotic deformity was 9° and improved by 15° after surgery. Follow-up films on 9 patients showed 5° of kyphosis correction loss. Mean anterior vertebral body height was 58% of normal before surgery. After surgery height was 89% of normal and at final follow-up, 78%. Conclusions. Segmental fixation of burst fractures with screws at the level of the fracture offers improved biomechanical stability. Theoretically, segmental fixation provides for additional fixation points that may aid in fracture reduction and kyphosis correction. This specific parameter is not evaluated in this study but will be an important outcome measure for a planned randomized controlled trial.

Minimally Invasive Spine Surgery

Paul C. McAfee, MD, MBA, Frank M. Phillips, MD, Gunnar Andersson, MD, PhD, Asokumar Buvenenadran, MD, Choll W. Kim, MD, Carl Lauryssen, MD, Robert E. Isaacs, MD, Jim A. Youssef, MD, Darrel S. Brodke, MD, Andrew Cappuccino, MD, Behrooz A. Akbarnia, MD, Gregory M. Mundis, MD, William D. Smith, MD, Juan S. Uribe, MD, Steve Garfin, MD, R. Todd Allen, MD, William Blake Rodgers, MD, Luiz Pimenta, MD, PhD, and William Taylor, MD

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.

Scientific basis of minimally invasive spine surgery: prevention of multifidus muscle injury during posterior lumbar surgery.

Study Design. Literature review. Objective. To describe the scientific basis of minimally invasive spine surgery as it relates to posterior lumbar surgery. Summary of Background Data. Minimally invasive spine (MIS) surgery is predicated on several basic principles: (1) avoid muscle crush injury by self-retaining retractors; (2) do not disrupt tendon attachment sites of key muscles, particularly the origin of the multifidus muscle at the spinous process; (3) use known anatomic neurovascular and muscle compartment planes; and (4) minimize collateral soft tissue injury by limiting the width of the surgical corridor. Methods. Literature review. Results. The conventional midline posterior approach for lumbar decompression and fusion violates these key principles of MIS surgery. The tendon origin of the multifidus muscle is detached, the surgical corridor is exceedingly wide, and significant muscle crush injury occurs through the use of powerful self-retaining retractors. The combination of these events leads to well-described changes in muscle physiology and function. MIS surgery is performed using table-mounted tubular retractors that focus the surgical dissection to a narrow corridor directly over the surgical target site. The path of the surgical corridor is selected on the basis of anatomic planes, specifically avoiding injury to the musculotendinous complex and the neurovascular bundle. Conclusion. With these relatively simple modifications to surgical technique, significant improvements in intraoperative blood loss, postoperative pain, surgical morbidity, return of function, among others, have been achieved. However, MIS techniques remain technically demanding and a significant complication rate has been observed during the initial learning curve of the procedures.

Treatment of Post-traumatic Ankle Arthrosis with Bipolar Tibiotalar Osteochondral Shell Allografts:

We report on tibiotalar osteochondral shell allografts for post-traumatic ankle arthropathy in seven patients. Average follow-up was 148 months (range, 85 to 198). Patients were evaluated by a questionnaire, SF-12 survey, ankle score, physical exam and radiographs. The ankle score increased from 25 preoperatively to 43 at latest follow-up (maximum score 100). SF-12 scores increased from 30 to 38 (Physical Component) and 46 to 53 (Mental Component). The failure rate was 42%. Four of seven patients reported good or excellent results. Five patients stated they would undergo a similar procedure again. Complications included graft fragmentation, poor graft fit, graft subluxation, and non-union. Follow-up radiographs demonstrated joint space narrowing, osteophytes, and sclerosis, even in cases with excellent clinical status. Fresh osteochondral shell allografting may provide a viable alternative for the treatment of post-traumatic ankle arthrosis in selected individuals.

Complications Associated With the Initial Learning Curve of Minimally Invasive Spine Surgery: A Systematic Review

BackgroundThere is an inherently difficult learning curve associated with minimally invasive surgical (MIS) approaches to spinal decompression and fusion. The association between complication rate and the learning curve remains unclear.Questions/purposesWe performed a systematic review for articles that evaluated the learning curves of MIS procedures for the spine, defined as the change in frequency of complications and length of surgical time as case number increased, for five types of MIS for the spine.MethodsWe conducted a systematic review in the PubMed database using the terms “minimally invasive spine surgery AND complications AND learning curve” followed by a manual citation review of included manuscripts. Clinical outcome and learning curve metrics were categorized for analysis by surgical procedure (MIS lumbar decompression procedures, MIS transforaminal lumbar interbody fusion, percutaneous pedicle screw insertion, laparoscopic anterior lumbar interbody fusion, and MIS cervical procedures). As the most consistent parameters used to evaluate the learning curve were procedure time and complication rate as a function of chronologic case number, our analysis focused on these. The search strategy identified 15 original studies that included 966 minimally invasive procedures. Learning curve parameters were correlated to chronologic procedure number in 14 of these studies.ResultsThe most common learning curve complication for decompressive procedures was durotomy. For fusion procedures, the most common complications were implant malposition, neural injury, and nonunion. The overall postoperative complication rate was 11% (109 of 966 cases). The learning curve was overcome for operative time and complications as a function of case numbers in 20 to 30 consecutive cases for most techniques discussed within this review.ConclusionsThe quantitative assessment of the procedural learning curve for MIS techniques for the spine remains challenging because the MIS techniques have different learning curves and because they have not been assessed in a consistent manner across studies. Complication rates may be underestimated by the studies we identified because surgeons tend to select patients carefully during the early learning curve period. The field of MIS would benefit from a standardization of study design and collected parameters in future learning curve investigations.

Posterior tibialis tendon tears

Gerling MC, Pfirrmann CA, Farooki S, et al. Posterior tibialis tendon tears: Comparison of the diagnostic efficacy of magnetic resonance imaging and ultrasonography for the detection of surgically created longitudinal tears in cadavers. Invest Radiol 2003;38:51–56. Rationale and Objectives.The optimal advanced imaging method for detection and characterization of posterior tibialis tendon (PTT) tears is unclear. The purpose of this study was to investigate the utility of ultrasonography (US) and MR imaging in the detection of surgically created PTT tears in cadavers. Materials and Methods.This was a prospective blinded study in which 16 fresh cadaveric foot and ankle specimens (3 men, 13 women; average age at death 83.9 years; age range 71–96 years) were scanned with both US and MR imaging before and after the surgical creation of 64 variable length longitudinal tears of the PTT. Ultrasonography was performed with a 12 MHz linear transducer with independent interpretations of static and dynamic studies separately by two blinded and experienced musculoskeletal radiologists. MR imaging was performed at 1.5 T with a standard transmit-receive extremity coil using axial, sagittal, coronal T1-weighted (TR 600, TE 20), and axial fast spin echo proton density and T2-weighted (TR 3000, TE 161/20, ETL 12) images. MR images were reviewed independently by two experienced musculoskeletal radiologists who were blinded to the status of the PTT. Results.Sensitivity, specificity, and accuracy of MR imaging in the diagnosis of PTT tears were 73%, 69%, and 72%, respectively. Dynamic US interpretation yielded values of 69% sensitivity, 81% specificity, and 72% accuracy. Static US interpretation was less reliable than dynamic interpretation, and the only significance of static imaging was a high specificity (94%) for detection of longitudinal tears. The positive predictive value (PPV) for MR imaging and US was 88% and 92% respectively, and the negative predictive value (NPV) was 46% for both MR imaging and US. Conclusion.Our results suggest that US and MR imaging perform at the same level for the detection of surgically created longitudinal PTT tears in a cadaveric model. US has a higher specificity compared with MR imaging.

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.