Introduction: The purpose of this study was to evaluate the design of a titanium/polyolefin artificial disc, assess its safety and measure outcome to determine if a RCT is justified.

Methods: All subjects entered this pilot study with one or two-level disc disruption at l4/l5 and/or l5/s1 and had disabling low back pain for at least 12 months. The diagnosis was confirmed by discography. Independent assessment included physical examination, VAS for pain, Low-Back Outcome Score (LBOS), Oswestry Disability Index (ODI), SF-36, lateral flexion/extension radiographs and MRI. This was carried out preoperatively, at 6 and 12 weeks, 6 months and at 1 and 2 years. Surgery was performed by an anterior retroperitoneal approach.

Results: Twenty-eight cases (14 males; average 41 years) were operated on during 1998–2000. Surgery was performed at L5/S1 in 19, L4/L5 in 5 and both levels in 4. Operating time averaged 130 minutes with 180mls average blood loss. There were no operative complications and average length of stay was 6 days. At 2 years there was 39% average improvement in vas, 15-point average improvement in ODI, 14 point average improvement in LBOS and improvement in 5 of the 8 SF-36 sub-scales. Complications included detection by plain radiographs of early partial displacement of implant in one case and late heterotopic calcification in another. Thin section helical CT, first carried out in early 2001, revealed polyolefin tears in 10 patients, four undergoing revision surgery since 2-year follow-up. CT revealed instances of osteolysis associated with polyolefin failure and hetero-topic bone formation not seen on plain radiographs.

Discussion: Although improvements in clinical outcome comparable to that reported with fusion were obtained at two years, detection of elastomer tears by thin section CT in 36% of patients indicated the prosthesis was not suitable for clinical use. This finding was unexpected given the results of prior extensive mechanical testing¹.

A prospective study to evaluate the design, outcome and complications of the AcroFlex titanium/polyolefin artificial lumbar disc replacement.

11 subjects with single-level discographically proven discogenic pain of at least six months duration and refractory to conservative treatment underwent Total Disc Replacement (TDR) using the AcroFlex TDR. Surgery was performed by an anterior retroperitoneal approach. The following outcome measures were recorded pre-operatively, at 6 weeks and 3, 6, 12 and 24 months: Visual Analogue Score (VAS), Oswestry Disability Index (ODI), Low Back Outcome Score (LBOS), and SF-36. Physical examination and radiological assessment (plain radiographs, flexion/extension views, cine-radiography) were performed at the same time intervals. Complications and reoperations were recorded.

11 patients were enrolled since April 1998 (7 male / 4 female). The mean age was 41. 3 years. All patients have been followed for a minimum of two years.

Surgery averaged 136 minutes with 143 mls blood loss. There were no operative complications. The average length of stay was 6. 1 days. The mean VAS reduced from 8. 8 to 4. 4 at two years. ODI improved from 51. 3 (mean) to 20. 9 (mean) at 24 months. The mean LBOS of 18. 4 improved to 47. 3 at two years.

Patients showed improvement in all subsets of the SF-36. Radiological examination confirmed a mean flexion/extension arc of 6. 6 degrees with restoration of native disc height. Adverse events included one disc expulsion (under radiological observation), one autofusion (F/E views still confirm movement) and one catastrophic rubber failure requiring revision to combined anterior/posterior interbody fusion. As a result of this case all patients underwent ultra fine cut CT scans. An additional 4 cases showed small anterior tears in the rubber and are currently asymptomatic.

The two-year outcome of the AcroFlex TDR is reported in 11 patients. Improvements in VAS, ODI, LBOS and all domains of the SF-36 were reported by 10 of 11 patients. Radiological outcome confirmed preservation of movement and restoration of disc height. Adverse events including disc expulsion, autofusion and rubber failure demand continued vigilance.

Introduction: The artificial disc consists of proprietary polyolefin rubber core bonded between two titanium endplates. It has been developed for the treatment of symptomatic disc degeneration with the aim of providing segmental stability and motion following wide disc space clearance. It was designed to have similar properties to a normal adult human intervertebral disc when working in conjunction with the retained anulo-vertebral tissues and the supporting musculoligamentous system.

Methods: Over 120 discs were used to biomechanically characterize the Device. Range of motion tests were designed and performed to measure the axial compression, torsional, and shear stiffness of the artificial disc and to compare this with the known values for the human lumbar disc. Pullout test was performed to evaluate the immediate and short-term stability of the inserted device by assessing the mechanical resistance to pullout or expulsion. To assess the ability of the implant to withstand average daily living loads throughout its predicted life, compression and compressive shear fatigue testing were performed.

Discussion: The device was found to replicate many of the physiologic characteristics of the in-vivo FSU. The quasi-static testing showed the device to have higher strength values than the highest in-vivo loads and displacements. Fatigue testing showed the smallest device endurance limit of 3,500N at ten million cycles.

The results demonstrate that the failure modes of the device contain sufficient safety margins to support the use of the device in a prospective clinical study.