In vivo deformation, surface damage, and biostability of retrieved Dynesys systems.

STUDY DESIGN: Retrospective retrieval analysis. OBJECTIVE: To evaluate wear, deformation and biodegradation within retrieved polycarbonate urethane (PCU) components of Dynesys systems. SUMMARY OF BACKGROUND DATA: The Dynesys Dynamic Stabilization System (Zimmer Spine) consists of pedicle screws (Ti alloy), polycarbonate urethane (PCU) spacers, and a polyethylene-terephthalate cord. METHODS: Seventeen retrieved (mean implantation: 2.5 years, range: 0.7-7.0 years) and 2 exemplar implant systems were available. Reasons for revision were persistent pain (16/17), infection (1/17), and/or screw loosening (11/17), with 1/17 case of implant migration. Optical microscopy, microCT, and scanning electron microscopy were conducted to evaluate PCU spacer wear and deformation. Attenuated total reflectance Fourier transform infrared spectroscopy was used to assess spacer surface chemical composition. RESULTS: Retrieved spacer components exhibited permanent bending deformation (mean: 4.3°, range: 0.0°-15.8°). We observed evidence of PCU spacer contact with pedicle screws, cords, and surrounding bony structures (74/75, 69/75, and 51/75 spacers, respectively). Relatively infrequent damage modes included PCU fracture (1/75 spacers) or cracking (2/75 spacers), as well as pedicle screw fracture (3/103 screws). PCU degradation products were identified in 10/75 spacers, which represented retrievals having significantly longer implantation times (mean: 4.3 years, range: 1.0-7.0 years). Of these spacers, 8/10 had degradation peaks identified along the side of the spacer where the material would have been in contact with bodily fluid. CONCLUSION: PCU spacers from retrieved Dynesys systems exhibited permanent deformation, focal regions of in vivo wear and surface damage. Chemical changes associated with PCU biodegradation were associated with longer-term retrievals. The most frequently observed complication was pedicle screw loosening, with 3 incidences of screw breakage in 2 patients. These retrieval data provide a crucial basis for developing in vitro tests to simulate in vivo damage and degradation of posterior dynamic motion preservation implants. Longer-term retrievals, as well as retrievals that include more recent design features (e.g., HA coating), will be useful to provide a greater context for the clinical implications of our short-term observations.

Surgeon and patient radiation exposure in minimally invasive transforaminal lumbar interbody fusion.

OBJECT: Minimally invasive transforaminal lumbar interbody fusion (TLIF) is an increasingly popular procedure. The technique involves use of fluoroscopy to assist with pedicle screw (PS) placement. The potential exists for both the surgeon and the patient to become exposed to significant amounts of radiation. The authors undertook this study to quantify the radiation dose to the surgeon and patient during minimally invasive TLIF. METHODS: The authors undertook a prospective study of 24 consecutive patients who underwent minimally invasive TLIF. All surgeries were performed by the senior author (R.K.B.), who used techniques previously described. The surgeon wore a radiation monitor under an apron-style lead shield at waist level, at an unshielded collar location, and as a sterile ring badge containing a thermoluminescent dosimeter on the dominant (right) hand ring finger. Dosimeter readings were obtained for each case. A total of 33 spinal levels were treated in 24 patients. All treated levels were between L3-4 and L5-S1. In all cases of 1-level disease, 4 PSs were placed, and in all cases of 2-level disease, 6 screws were placed. RESULTS: Mean fluoroscopy time was 1.69 minutes per case (range 3.73-0.82 minutes). Mean exposure per case to the surgeon on his dominant hand was 76 mRem, at the waist under a lead apron was 27 mRem, and at an unprotected thyroid level was 32 mRem. Mean exposure to the patient's skin was 59.5 mGy (range 8.3-252 mGy) in the posteroanterior plane and 78.8 mGy (range 6.3-269.5 mGy) in the lateral plane. CONCLUSIONS: To the authors' knowledge, this is the first study of radiation exposure to the surgeon or patient in minimally invasive TLIF. Patient exposures were low and compare favorably with exposures involving other common interventional fluoroscopically guided procedures. Surgeon exposures are limited but require careful monitoring. Annual dose limits could be exceeded if a large number of these and other fluoroscopically guided procedures were performed.

Intraoperative electromyography monitoring in minimally invasive transforaminal lumbar interbody fusion.

OBJECT: Minimally invasive transforaminal lumbar interbody fusion (TLIF) is an increasingly popular method for achieving lumbar decompression and fusion. The procedure is technically more demanding than open fusion, with correspondingly more theoretical risk of complication. The authors describe the use of intraoperative electromyography (EMG) as an adjunct to surgery to reduce the risk of complications. METHODS: Between August 2005 and April 2006, 25 consecutive patients underwent minimally invasive TLIF in which a total of 105 pedicle screws were placed. Intraoperative EMG was performed and included passive recordings during decompression and interbody graft placement, as well as active recording during the placement of the pedicle access needle and testing of the pedicle tap. A uniform protocol for active monitoring was used, with the pedicle access needle set at 7 mA. To assess hardware placement, all patients underwent postoperative radiography and 20 underwent postoperative computed tomography (CT) scanning. In no patient did the authors observe significant EMG activation during decompression. In five cases, intermittent nerve root firing was noted after the interbody graft was placed, but this did not correlate with any postoperative deficits. Using the active stimulation protocol, 76.2% of screw placements required one or more changes to the trajectory of the pedicle access needle. With successful placement of the pedicle access needle, in all 105 screws, the pedicle tap nerve root stimulation threshold was greater than 15 mA. Postoperative radiography was performed in all patients and CT scanning was performed in 20 patients (with 85 screws being placed). Postoperative imaging revealed only three cases of pedicle breach. In all cases, the breach was at the lateral wall of the pedicle and not thought to be clinically relevant. CONCLUSIONS: A continuous stimulation pedicle access needle alerts the surgeon to incorrect medial trajectories and may lead to safer pedicle cannulation. As a result of electrophysiological feedback, the pedicle access needle trajectory was altered in 76.2% of the reported cases. The use of the authors' protocol resulted in a 0% incidence of clinically relevant malpositioned hardware and a low overall neurological complication rate. Intraoperative nerve root monitoring is a useful adjunct to minimally invasive TLIF.

Resorbable anterior cervical plates for single-level degenerative disc disease.

OBJECTIVE: Resorbable spinal implants have generated increasing interest. There are minimal data in the literature on the use of resorbable anterior cervical plates. The purpose of this study is twofold. First, we present clinical outcomes of anterior cervical decompression and fusion (ACDF) for single-level degenerative disease using the first commercially available resorbable cervical plate. Second, we compare x-ray outcomes, including pseudarthrosis rates and angulation of healing, between patients receiving resorbable plates and a control group of patients receiving titanium plates. METHODS: Twenty-four consecutive patients treated with single-level ACDF using a resorbable anterior cervical plate were prospectively evaluated with minimum 12-month follow-up (mean, 15 mo). As a control group, 93 consecutive patients treated with single-level ACDF using a titanium plate with minimum 1-year follow-up were assessed. RESULTS: Overall outcomes were good in the resorbable plate group at a mean 15 months follow-up. Neck Disability Index scores decreased from a mean of 64.0 to 20.1% at follow-up (P < 0.001). Numerical Rating Scale pain scores decreased from a mean of 8.0 preoperatively to 2.9 at follow-up (P < 0.001). Three cases of pseudarthrosis [three of 24 (12.5%)] were identified in the resorbable plate group compared with two cases in the control group [two of 93 (2.2%)]. This difference was significant (P = 0.047). Angulation was measured on final x-rays using Cobb angles. Patients in the resorbable plate group healed with a mean 4.52-degree kyphosis, whereas patients in the titanium plate group healed with a mean 2.41-degree lordosis (P < 0.001). CONCLUSION: This report describes, to our knowledge, the first series of patients to undergo ACDF with the first commercially available resorbable cervical plate. Healing in kyphosis and pseudarthrosis was significantly more likely with resorbable implants as compared with the use of titanium plating. Although ultimate clinical outcomes were good, x-ray outcomes may be better with the use of titanium plating.

The natural history of untreated skull base meningiomas.

BACKGROUND: This study was undertaken to determine the natural history of untreated skull base meningiomas. Although there are reports on the natural history of meningiomas, most series contain only a few cases of meningiomas involving the skull base. Natural history information is important when recommending treatment and evaluating results. METHODS: The case records and imaging studies of 40 patients with skull base meningiomas from a single practice were reviewed. These patients either did not receive treatment or there was a long delay from the onset of symptoms to the initiation of treatment. RESULTS: Skull base meningiomas may be very indolent tumors. Patients with these tumors have lived long, comfortable, and relatively productive lives without treatment or with delayed treatment. Impairment may be much less than imaging studies would suggest. CONCLUSION: The authors are not advocating that skull base meningiomas not receive treatment. However, their unique experience with this group of patients provides convincing evidence that the indolent behavior of some skull base meningiomas must be considered when recommending various surgical and radiation procedures and interpreting their results in relationship to the functional outcome of the patient.