Steven I. Reger

Cortisone-induced Intrafemoral Head Pressure Change and its Response to a Drilling Decompression Method

Intramedullary head pressure changes were studied in a group of cortisone-treated New Zealand white rabbits. In addition to femoral head pressure measurements, serum cholesterol levels were followed serially and pathologic specimens from cortisone-treated femoral heads were obtained. Surgical decompression by a drilling method was performed and pressures were remeasured at four weeks following decompression. Persistent increases in cholesterol levels and in intrafemoral head pressures were noted, particularly from six to eight weeks after initiation of steroid treatment. Cholesterol levels were elevated to four times control values and femoral head pressures averaged 2 1/2 times the control values (60 cm of water vs 24.6 +/- 5.5 cm of water). Surgical decompression by drilling effectively reduced the once elevated pressures to control values. A relationship between increased fat cell size, increased intramedullary pressure and decreased femoral head blood flow in steroid-treated rabbits is postulated. The applicability of these data to humans with steroid-induced ischemic necrosis of the femoral head has yet to be demonstrated.

Experimental femoral fracture immobilized by rigid and flexible rods (a rabbit model).

Experimental fractures in rabbits demonstrated that flexible intramedullary rod fixation of the midshaft long bone fracture enhances the fracture healing and may promote early mobility. When compared to rigid fixation, both excess flexibility and excess rigidity inhibited optimal fracture healing. The occurrence of angulation deformities with plastic rods also demonstrate the disadvantage of a very flexible rod.

The use of methylmethacrylate cement as an instantaneous fusion mass in posterior cervical fusions: a canine in vivo experimental model.

The authors previously predicted the failure of posterior cervical fusions utilizing methylmethacrylate cement as an instantaneous “fusion” mass, based on research using an in vitro canine experimental model. This report describes the results of in vivo canine studies on the same subject. Three groups of dogs had application of a posterior C4-C5 20-gauge cerclage wire and autologous lilac crest bone graft; application of a posterior C4-C5 20-gauge cerclage wire and methylmethacrylate cement; or application of a C4-C5 20-gauge cerclage wire only. This group represented the control group. The dogs were allowed to live for 3 months postoperatively, at which time they were killed and their spine fusions studied radiologically, mechanically, and histologically. Five of the bone fusions united solidly radiologically. Their flexion stability was statistically superior to the others. Histologic studies confirmed solid union of the fusion mass to the underlying bone. Four of the six methylmethacrylate fusions demonstrated cerclage wire fracture and methacrylate-bone separation by the second postoperative month. At the time the dogs were killed, their flexion stability was statistically inferior to the bone fusions and tended to be inferior to the controls as well. Histologically, fibrous tissue was noted to have grown between the methacrylate “fusion” mass and the underlying bone. This work provides a mechanical explanation for the well-known success of the traditional bony fusion. It further supports our original prediction regarding the failure of methylmethacrylate “fusions”.

Femoral head blood flow in long-term steroid therapy: study of rabbit model

ABSTRACT: Using a rabbit model, previous studies showed steroid-induced hyperlipidemia with subsequent fatty embolization of the subchondral arteries and hypertrophy of the marrow fat cells, followed by elevation of femoral head pressure from the normal level of 25 cm to nearly 60 cm H2O after eight weeks of treatment. This has led us to believe that pressure changes lead to decreased blood flow in the femoral head. In our study of 22 New Zealand white adult rabbits, weighing an average of 4.0 kg, 14 received a weekly dose of 12.45 mg of methylprednisolone (Depo-Medrol), and eight served as control. Femoral head blood flow was established using the radioactive microsphere technique. Control and cortisone-treated rabbits had femoral head blood flow measured 6, 8 and 10 weeks after treatment. The average blood flow in the control femoral, heads averaged 0.2039 ± 0.076 ml/min/gm, with no difference in the left side and the right side. In the treated group, the average blood flow at ten weeks was 0.162 ± 0.039 ml/min/gm on the right and 0.164 ± 0.037 ml/min/gm on the left, which was significantly different. This is parallel to unpredictable clinical findings in human beings.

Semirigid rod fixation for long-bone fracture

Experiments on 119 New Zealand rabbits were performed to measure healing strength of femurs. Four pairs of normal rabbit femurs were also studied in vitro by use of a four-point bending test to measure bending rigidity. Rabbit-sized octagonal cross-sectioned intramedullary rods with 12%, 20%, 50%, 75%, and 100% of femoral bending rigidity were specially prepared. For each animal, one intramedullary rod was implanted in an osteotomized femur through a standard lateral approach, with the other femur serving as a control. The results from the 12% rigidity rods were widely variable. In the 20% and 50% rigidity groups, the callus diameter was increased over the 75% rigidity group through all phases of healing. The torque ratio in this group increased up to eight weeks and then converged to the ratios of the other rigidities. The 75% rigidity group had shown increase of angular displacement up to eight weeks, while the 20% and 50% rigidity groups continued to improve and reached control strength at 12 weeks of treatment. Furthermore, the 20% and 50% groups absorbed more energy to failure in the later stage of healing than the 75% and 100% rigidity groups. Rods of 20% and 50% rigidity stimulate external callus formation and improve the stability of healing bone.

Reconstruction of the cervical spine following anterior vertebral body resection: a mechanical analysis of a canine experimental model.

In some quadriplegic patients whose acute spinal injury consists of a vertebral body fracture with bony retropulsion anteriorly and ligamentous damage posteriorly, It is appropriate to perform a decompression of the injured spinal cord by excision of the fractured body. Reconstruction of such a spine to achieve immediate and ultimate stability can be quite difficult. This is a report of mechanical testing using a canine in vitro experimental model to simulate different general types of reconstruction systems applicable to the spine destabilized as above. The results explain some of the previously noted failures of the traditional anterior strut grafting procedures. In addition, one of the reconstruction systems tested (anterior and posterior tension bands compressing an anterior strut graft into place) appeared to have sufficient immediate stability to eliminate the need for a halo-vest in the post-operative period. If such a reconstruction system were to prove clinically applicable, it would be quite beneficial to the quadriplegic patient in his rehabilitation.

The safety of cement fixation in the cervical spine. Studies of a rabbit model.

Immediate postoperative ambulation of patients with terminal neoplastic disease of the cervical spine can be permitted by vertebral stabilization with wire reinforced with polymethylmethacrylate. Precautions are necessary to avoid a temperature rise during cement polymerization and prevent vasular or nerve damage. Experiments on 20 rabbits and 2 human cadavers indicate that temperature at the cement-lamina interface can rise as high as 90 degrees while temperature under the intact lamina measures only 50 degrees. Precooling and insulation with Gelfoam may prevent heat injury to spinal cord.

Comparative strength of anterior spinal fixation with bone graft or polymethylmethacrylate. Experimental operations and observations on dogs.

In 23 dogs, polymethylmethacrylate (PMMA) was safely implanted in the region of the spinal cord. The dogs were treated by bone or cement fixation after anterior excision of a single cervical vertebral body. Fixation was compared at 0-18 weeks, with control values obtained from 11 dogs. Anterior cement fixation did not displace after 18 weeks. The anterior bone graft appeared to achieve adequate fixation strength around six weeks. The strength of the bone-graft fixation increased, while that of cement fixation decreased somewhat with time. At all time intervals, the ratio of energy absorption to structural failure was somewhat higher with bone graft than with PMMA.

A biomechanical analysis of posterior cervical fusions using polymethylmethacrylate as an instantaneous fusion mass.

Three posterior fusion reconstruction procedures for a canine experimental model of the C4-C5 bilateral facet dislocation are herein mechanically tested. The two procedures utilizing polymethylmethacrylate have increased angular stiffness as compared with the normal posterior soft tissue structures and the reconstruction procedure utilizing wire alone. Their ultimate moments are not significantly greater. These findings lead us to predict an increased incidence of fixation failure when polymethylmethacrylate acrylic cement preparations are used for instantaneous posterior “fusions” as compared with the more traditional wire and bone graft procedure.

Comparative strengths of various anterior cement fixations of the cervical spine.

In vivo study, using cadaveric dog cervical spine, was performed in which the middle segment of the vertebral body was removed producing anterior instability. Eleven various methods of stabilization utilizing polymethylmethacrylate and other fixation devices were employed, and their static strength in hyperextension was tested and compared. A portion of the same dogs cervical spine in each case was used as a control. The results indicate that all fixation methods failed to regain the normal structural strength in extension. Cement with wire or chain methods of fixation were superior to other methods in this study. Fixation rigidity approaching rigidity of the normal spine appeared to be a significant factor determining the strength of the reconstructed cervical spine. Combined anterior and posterior fixation did not provide further strength, although it did increase the rigidity of the fixation.