Intervertebral spacer as an adjunct to anterior lumbar fusion. Part I. Design, fabrication, and testing of three prototypes.

Failure of anterior lumbar fusions are common, because the bone graft is required to provide mechanical stability during creeping substitution and replacement with host bone. Support of the interspace with mesh, cages, plates, and rods results in an improved rate of fusion. The objective of this study was to develop an anterior interbody implant to stabilize adjacent vertebral segments during spinal fusion. Three prototypes of an intervertebral spacer for anterior lumbar stabilization were designed, fabricated, and tested in vitro. The implants were inserted vertically between adjacent vertebral bodies in fresh frozen cadaver swine and baboon spines after disc excision and vertebral body preparation. In vitro cyclic testing of the three prototypes implanted in porcine and baboon spines to 100,000 cycles showed no displacement of the implant at 560-N axial and 16-Nm torsional loading. Three-point bending cyclic fatigue testing of the porous coated cylindrical implants (prototype 3) showed a maximum strength of 9,700 N in axial compressions. Analysis of the motion profiles at the site of implantation confirmed less axial displacement at the implant level compared with the uninstrumented levels above and below, but similar torsional displacements. Biomechanical testing of the three prototypes of anterior implants as well as radiographic, microstructural, and motion analysis confirmed implant stability and structural integrity in vitro. Based on these findings, implantation of a porous coated implant in baboons was undertaken as described in Part II (Nasca et al., this issue).

Intervertebral spacer as an adjunct to anterior lumbar fusion. Part II. Six-month implantation in baboons.

Failure of interbody fusions in the lumbar spine are common due to reliance on the graft for structural support during healing by creeping substitution. Support of the interspace with an implant should result in improved fusion success. The objective of this study was to evaluate the stability of the implant in vivo and its potential as an adjunct to promote interbody arthrodesis. Prototype 3, a porous coated intervertebral spacer with extension lugs made of Ti-6A1-4V, was implanted vertically between adjacent lumbar vertebrae anteriorly in four baboons undergoing anterior interbody fusion. The animals were allowed freedom of activity for 6 months before being killed. A transperitoneal approach was made exposing the L4-L5 or L5-L6 interspace. At time of killing, clinical evaluation of the implant-vertebral body construct showed stability to manual stresses applied in extension, flexion, and rotation. Serial radiographs taken during the 6 months of implantation showed no change in position or displacement of the implants. Axial and torsional cyclic loads were applied to each spine at 1 cycle/s for 20,000 cycles. Statistical analysis of the motion profiles for intact and implanted spines demonstrated no significant difference in axial or rotational displacements at the arthrodesis level or adjacent unoperated levels, L1 and L4. The in vivo 6-month study in baboons confirmed implant stability and maintenance of disc space height. Variable osseous healing was noted. Release of plasma spray beading may have resulted from improper application on the implant or micromotion within the construct. A better method to mechanically interlock the plungers is being studied. Clinical trials based on this work appear justified.

Evaluation of cryopreserved bone and synthetic biomaterials in promoting spinal fusion.

The purpose of the study was to evaluate the use of cryopreserved allograft bone and tricalcium phosphate in promoting spinal fusion. Nine 20-30 lb swine underwent posterior spinal fusion at T5-T6, T13-T14, and L2-L3. Autogenous bone, cryopreserved allograft bone, or equal parts of allograft bone and tricalcium phosphate were added to the decorticated posterior elements. A total of 27 sites were prepared for fusion. The spines were retrieved at 6 months and evaluated for integrity and stability of the fusion sites by clinical examination, three-point bending tests, multiplanar radiographs, and undecalcified tetracycline-labeled and decalcified histologic sections. The nine sites that received autogenous bone were solidly fused. There were one clinical and two radiographic nonunions in the nine sites that received cryopreserved allograft bone. Sites that received a mixture of allograft bone and tricalcium phosphate demonstrated slight motion at two locations and radiographic evidence of fusion at all levels. The extent and degree of fusion was not site-specific. Three-point bending analysis did not demonstrate a significant trend as to site or materials specificity. No adverse histologic response was noted. Histologic sections and tetracycline labels confirmed abundant new bone formation at all sites at 6 months. Although autogenous bone remains the gold standard for use in spinal arthrodesis, this study demonstrates the value of cryopreserved allograft bone alone and in combination with tricalcium phosphate in promoting spinal fusion.

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.

Multiaxis cyclic biomechanical testing of Harrington, Luque, and Drummond implants.

The performance characteristics of Harrington-Moe distraction rods, paired wired Luque rods and Drummond's system were evaluated and compared when subjected to nondestructive cyclic, multidirectional biomechanical testing. Twelve fresh, frozen swine spines with intact facet joints and anterior and posterior ligamentous complexes were instrumented and tested in a specially designed pneumatic testing machine. The instrumented spines were subjected to 207 kN/m2 compression and 49 N-m torsion. Each spine was cycled 28,000 times at 1 cycle per second. Linear and angular displacements were determined by digitizing photographs and video tapes made during testing. A computer program developed and refined for the project was used to complete the data analysis. Approximately 540 items of angular and linear displacement data were collected for each spine. Inspection of the spines after cyclic, multidirectional testing revealed no change in their osteoligamentous integrity compared with pre-testing. Pre- and post-testing radiographs showed no evidence of osseous failure, hook dislodgement or wire breakage. Erosion of laminal bone at the Harrington hook attachment sites was observed. Displacement of the Harrington hooks was seen during off-axis compression-torsion testing. Fretting and deposit of metal wear debris occurred between the sublaminal wires and "L" rods. There was no evidence of loosening of either the Drummond or Luque implants or fatigue failure of any component. Analysis of the linear and angular displacement data showed that the Luque and Drummond instrumented spines displaced less in axial compression, off-axis compression and off-axis compression-torsion than the single Harrington-Moe distraction rod.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthetic biomaterials for spinal fusion.

Calcium hydroxylapatite (HAP), tricalcium phosphate (TCP), and Bioglass (BG) were implanted in the spines of dogs to determine their potential in augmenting and enhancing spinal fusion. HAP and TCP showed continuous bone to biomaterial interfaces of varying degrees. Trabecular bone surrounded and incorporated the particulate hydroxylapatite. Tricalcium phosphate ceramic showed little evidence of resorption. The glass particulate, BG, showed a thin, fibrous encapsulation with some adjacent bony trabeculae. Decortication and autogenous bone enhanced incorporation. This multivariant initial study showed that trends found for the biomaterials implanted independently were similar to those used in combination. No adverse tissue reactions were noted for the combination of materials.

Rationale for spinal fusion in lumbar spinal stenosis.

In order to define the indications for spinal fusion in patients undergoing decompression for lumbar spinal stenosis, 114 patients surgically treated were reviewed. Follow-up was 24 to 108 months. Patients were grouped into four categories: 15 with lateral recess stenosis, 45 with central-mixed stenosis, 43 with stenosis following prior lumbar surgery(s), and 11 with scoliosis and spinal stenosis. Only two patients with lateral recess stenosis underwent fusion with fair results. Approximately one-third of those with central-mixed stenosis required a fusion. Results were good in 70%. In those with stenosis following prior lumbar surgeries, although not statistically significant, those who had concomitant decompression and arthrodesis had a better outcome than those in whom decompression only was done. Patients with scoliosis and stenosis had decompression for significant motor and reflex deficits and fusion over the length of their major curves. Patients having decompression for lumbar stenosis with degenerative spondylolisthesis, isolated disc resorption with degenerative facet joints, intervertebral disc disease with instability, and those with scoliosis with multidirectional instabilities benefit from concomitant spinal fusion.

Unicameral bone cyst of the tibia complicated by genu valgum.

Progressive tibia valga occurred in an adolescent girl after curettage and allografting of a large unicameral bone cyst of the tibia. The valgus deformity was corrected by a closing-wedge osteotomy, which resulted in a good functional and cosmetic result. The progressive valgus deformity might have been caused by stimulation of overgrowth of the medial tibial metaphysis.

Transthoracic exposure for anterior spinal surgery.

The anterior approach to the vertebral column is used to treat fractures, spinal deformities, and destructive lesions secondary to tumor or infection. The thoracic surgeon, working with orthopedic surgeons and neurosurgeons, is uniquely qualified to provide surgical exposure expediently and assist in postoperative care. Forty-five patients with spinal deformities secondary to trauma (18 patients), congenital anomalies (16 patients), neoplastic disease (7 patients), and inflammation (4 patients) were treated by a transthoracic (37 patients) or thoracoabdominal (10 patients) anterior approach to the vertebral column. Two patients had subsequent operations. Free and vascularized rib grafts were used for stabilization and fusion with good results and few complications (8 patients). These results indicate that interspecialty cooperation results in expedient surgical exposure and good postoperative care.

Harrington-Bobechko instrumentation in the treatment of scoliosis: a preliminary report.

A dual hook system devised by Bobechko for use with Harrington and Moe distraction rods to eliminate postoperative orthotic support was employed in the surgical treatment of 57 patients with progressive scoliosis. Mean follow-up time was 33 months (range 15 to 56 months). Average correction was 51%, with an average loss of correction of 8 degrees. Five patients required reinstrumentation, two of whom had dislodgement of both upper hooks during the first postoperative week; the other three had rod breakage which occurred 3 to 20 months after surgery. The results indicate that the dual hook system provides greater security of fixation than a single hook construct, but has the disadvantage of bulkiness, making it prominent in thin patients. Postoperative protection with bracing for 6-9 months continues to be recommended to guard against instrument failure.

The use of freeze-dried allografts in the management of global rotator cuff tears.

Massive or global tears of the rotator cuff with loss of tendon substance and retracted cuff remnants pose a challenging surgical and rehabilitation problem. In seven patients, global cuff tears were reconstructed with freeze-dried rotator cuff allografts. Five of the seven patients had other significant debilitating medical problems. Three patients had received four steroid injections, one patient had six injections, and the remaining three patients had two injections in or about the affected shoulder. Five of the seven patients had their pain relieved. Only two patients had significant improvement in shoulder function after anterior acromioplasty and allografts. Freeze-dried rotator cuff allografts do not appear to be of significant value in the surgical management of chronic massive rotator cuff tears.

Surgical management of isthmic spondylolisthesis.

In order to determine the factors that produce a successful outcome in the surgical management of spondylolisthesis, 44 consecutive patients who underwent surgical treatment were reviewed. Preoperative evaluation was directed towards determining the presence or absence of nerve root compression. The patients without nerve root compression typically underwent fusion alone, while those with evidence of nerve root compression had decompression and fusion. After an average 36-month follow-up, results were good in 78%, fair in 18%, and poor in 4%. There were five complications, all of which were successfully managed. Two patients required reoperation for pseudoarthrosis. An eventual 100% fusion rate was achieved. Results in the patients with Grade III or Grade IV olisthesis were nearly equivalent to the less severe cases. Nine patients received allograft, with a fusion rate equal to the 32 receiving autogenous bone. This study suggests that in patients who have isthmic spondylolisthesis with mechanical lumbar symptoms only, or in patients with low-back pain with radicular extremity pain caused by nerve root irritation, fusion alone is indicated. In patients with documented radiculopathy resulting from nerve root compression, decompression in addition to fusion is indicated.

Surgical management of lumbar spinal stenosis.

Eighty consecutive patients with lumbar spinal stenosis surgically treated during a 5-year period by the author were reviewed. Patients were placed in the following categories: lateral spinal stenosis (10), central-mixed stenosis (29), spinal stenosis after laminectomy and/or fusion (32), and spinal stenosis with degenerative scoliosis (9). Contrast-enhanced computed tomographic (CT) scans were helpful in determining the levels requiring decompression. However, in the multiply operated patient, contrast-enhanced CT scans were misleading in six patients. Patients with lateral spinal stenosis were treated with unilateral laminectomy and partial facetectomy. The 29 patients with central-mixed stenosis underwent decompressive laminectomy and bilateral facetectomies. Six fusions were done. In the nine patients with spinal stenosis and scoliosis, concaveside partial facetectomies and laminectomies were done as well as spinal fusions. The 32 patients with spinal stenosis after previous laminectomy and spinal fusions were the most difficult group to analyze, and their treatment was the least standardized. There were 19 good, eight fair, and five poor results in those who had undergone previous surgery. Fifty-seven of the 80 patients (71%) experienced a good result from their surgical treatment.

Transient osteoporosis.

Six hips in four patients with hip pain, limited range of motion, no antecedent trauma, and normal laboratory studies demonstrated roentgenographic evidence of periarticular osteoporosis. The subchondral cortex was attenuated. There was little or no cartilage loss. The preliminary diagnosis was transient osteoporosis (migratory osteolysis, regional osteoporosis). This problem is most frequently seen in the hip joint in two population groups: men near 40 and women in the third trimester of pregnancy. Other joints may be involved and the process may regress in one joint but recur in another. Diagnosis is based on typical clinical and roentgenographic observations including an aspiration of sterile joint fluid. Radioisotope scanning may be helpful. Bone or synovial biopsy sampling is not necessary. Proper management requires accurate diagnosis and conservative treatment of a cooperative patient. Pain may persist for six months or longer. Treatment consists of analgesics, protection against stress fractures, and physical therapy for prevention of contractures.

Anatomical observations on the acromioclavicular joint and supporting ligaments.

An anatomical study of the acromioclavicular (AC) joint and its supporting ligaments was performed using both macroscopic and microscopic methods. The project used 63 cadaver shoulders of unknown ages. Fifty-three joints were used for macroscopic and 10 for microscopic study. The data consisted of micrometer measurements of the dimensions of the extrinsic and intrinsic ligaments of the AC joint; measurement and description of the intraarticular meniscus and the superior and inferior capsular ligaments of the AC joint; and the anatomical course and relationship of the coracoacromial (CA) ligament to the supporting ligaments of the AC joint and a description of its insertion on the acromion process. The following observations were made: The coracoclavicular ligament, especially the trapezoid ligament, provides significant soft tissue restraints to upward displacement of the clavicle. A complete AC joint disc was found in only one, meniscoid discs in 25, remnants of discs in 16, and no discs in 11 of the 53 joints studied macroscopically. (No age correlation was made since the ages of the cadavers were unknown). The CA ligament interconnects with the inferior capsular ligament of the AC joint as well as with the coracoclavicular and coracohumeral ligaments. It also has a broad area of insertion on the inferior acromial surface. The CA ligament appears to function as a buffer between the acromion and the rotator cuff, and to provide support for the AC joint. Transection of the CA ligament may result in loss of this buffering function.

Use of cryopreserved bone in spinal surgery.

One hundred fifty-two consecutive spinal fusions were performed over a 4-year period in 143 patients. Autogenous bone was used in 62 patients and frozen cryopreserved bone in 90. A variety of anterior and posterior procedures with and without instrumentation were performed. The percentage of successful arthrodesis was 87 in those who received autogenous bone, and 86.6 in those who received allograft bone. Thirty-four spinal fusions were surgically explored. Histologic evaluation of the bone taken at the time of surgical exploration showed viable osteocytes laying down osteoid, woven and lamellar bone, and no inflammatory or foreign body reaction. The authors conclude that cryopreserved bone, harvested and processed as described, is advantageous, safe, and results in a rate of bone union comparable to that of autogenous bone.