Richard H. Brown

Telemetric force measurements across the hip after total arthroplasty

A telemeterized total hip prosthesis was implanted in one patient and force-data were obtained. Thirty-one days postoperatively, the magnitude of the joint-contact force during double-limb stance was 1.0 times body weight. During ipsilateral single-limb stance the joint-contact force was 2.1 times body weight, and during the stance phase of gait the peak force typically was 2.6 to 2.8 times body weight, with the resultant force located on the anterosuperior portion of the ball. During stair-climbing, the force was 2.6 times body weight. At peak loads, the angle between the resultant force and the axis of the neck was 30 to 35 degrees and that between the resultant force and the plane of the prosthesis was 20 degrees. During stair-climbing or straight-leg raising, the out-of-plane orientation of the resultant force increased substantially. These data provide information concerning the forces that must be sustained by prosthetic hip joints during a number of common activities of daily living within the first month after implantation. The results also provide insight into the progression of early recovery and demonstrate the variety of forces that are generated during this period.

Spinal cord monitoring during operative treatment of the spine.

Twenty-six orthopedic and 8 neurosurgical patients undergoing spine surgery had spinal cord monitoring before, during, and after operation using somatosensory, averaged cortical evoked responses. Although no inherent risks were apparent in the technical application of this form of spinal cord monitoring, there are limitations that have yet to be evaluated. Classically, somatosensory evoked responses have been considered a function of the posterior columns; however, the results of these studies indicate that more than the function of the dorsal columns alone can be evaluated with this technique. In addition, much remains to be learned regarding the changes in signals noted and the corresponding clinical conditions. Techniques more sophisticated than visual evaluation of response patterns must be established and more sophisticated methods of analysis must be developed. Despite the need for more knowledge of the nature of this system and the correlation between evoked responses and clinical conditions, the system has proved to be effective and to have great potential for improving spine and spinal cord surgery.

Spinal analysis using a three-dimensional radiographic technique.

Abstract This paper presents an in vivo clinical method for describing the three-dimensional configuration of spinal segments. The technique is based upon the characterization of each vertebral segment by four points in three-dimensional space using bi-plane roentgenography and subsequent analysis by computer aided descriptive geometry techniques. Tests were conducted to determine the systems accuracy and repeatability. The results of these tests using both a calibrated mechanical model of the spine and actual in vivo spine evaluations are presented.

Cortical Evoked Potential Monitoring: A System for Intraoperative Monitoring of Spinal Cord Function

This report describes the intraoperative use of a somatosensory cortical evoked potential system to monitor spinal cord function during 300 orthopedic surgical procedures. This system requires sophisticated equipment and the establishment of normative data but has performed well with no false negatives noted. The most frequent technical problem encountered was the effect of medications and anesthetic agents, some of which impair the evoked response significantly. In this series of 300 cases, three neurologic deficits were documented intraoperatively and confirmed postoperatively. There were four cases in which changes in evoked potentials led to change in operative procedure, with no subsequent neurological deficit. In the remaining cases, the monitoring indicated no neurologic problems during surgery, and none were noted postoperatively.

Effects of fentanyl and morphine on intraoperative somatosensory cortical-evoked potentials.

We compared the effects of morphine and fentanyl in the presence of 60% N2O on somatosensory cortical-evoked potentials (SCEP). Both drugs were administered by intravenous bolus (n = 12) and infusion (n = 20) techniques. SCEPs were recorded preoperatively and intraoperatively in 32 patients undergoing corrective surgery for scoliosis. Records were taken at the contralateral cerebral cortex by individual stimulation of the posterior tibial nerves at the ankle. Both drugs increased the latencies of the N1, P2, and N2 peaks and affected the peak-to-peak amplitudes of the primary complex. Intravenous bolus injections and continuous infusions of equianalgesic doses produced similar effects. The increase in N1 latency was significantly greater (morphine, P < 0.05; fentanyl, P < 0.01) with the bolus than with the infusion technique. The doses of morphine and fentanyl given by bolus injections were 1/3 times and 3 1/3 times greater than doses given by infusion. Changes in latency were more consistent than changes in amplitude. Both fentanyl and morphine increase latencies while affecting amplitudes unpredictably. Equianalgesic doses of fentanyl and morphine have similar effects on SCEP latencies. Low-dose continuous infusions of narcotics depress SCEPs less than intermittent bolus injections.

DELIBERATE HYPOTENSION FOR SPINAL FUSION: PROSPECTIVE RANDOMIZED STUDY WITH EVOKED POTENTIAL MONITORING

Twenty-four patients requiring spinal fusion with Harrington rod instrumentation were studied prospectively to determine the effects of moderate hypotension on blood loss, operating conditions, operating time and spinal cord function. Hypotension reduced blood loss and improved operating conditions but did not shorten operating time. Five patients had alterations in somatosensory cortical evoked potentials after straightening of the spine that prompted us to reverse hypotension (when present) and haemodilution, and then to do wake-up tests. All wake-up tests were normal and all evoked potential alterations resolved during operation. Hypotension seems unlikely to increase the risk of neurological damage if spinal cord function is monitored. Our findings suggest that patients subjected to spinal fusion need not be awakened during operation for testing of cord function provided somatosensory evoked potentials are monitored and remain stable.RésuméL’emploi d’une hypotension contrôlée modérée (maintien de la pression systolique entre 80 et 90 torr) au cours de fusions vertébrales par tige de Harrington a été étudié de façon prospective. Les paramètres étudiés étant le volume des pertes sanguines opératoires, la qualité du champ opératoire, la durée de l’intervention et le maintien de la fonction médullaire. Les vingt quatre patients inclus dans cette étude étaient assignés au hasard au groupe témoin ou au groupe opéré sous hypotension contrôlée. Nous avons observé des pertes sanguines moindres ainsi qu’une qualité supérieure du champ opératoire chez les patients opérés sous hypotension contrôlée. La durée de l’ intervention n’était cependant pas raccourcie. Cinq des patients ont présenté des modifications des potentiels corticaux induits par stimulation somato-sensorielle; dans ces cas la pression était ramenée à la normale lorsque l’hypotension contrôlée était utilisée, l’hémodilution était corrigée et le malade était éveillé en cours de chirurgie pour vérifier si la motricité volontaire des membres inférieurs était conservé. Tous les tests d’éveil se sont avérés normaux et les potentiels corticaux évoqués se sont corrigés en cours d’intervention. L’emploi d’hypotension contrôlée ne semble pas susceptible d’augmenter le risque de dommages neurologiques si le maintien de la fonction de la moelle épinière est surveillé en cours de chirurgie.Nos résultats suggèrent que les malades soumis à une fusion vertébrale n’ont pas à être éveillés en cours d’intervention pour vérification du maintien de leur fonction médullaire en autant que l’on effectue le monitoring per-opératoire des potentiels corticaux induits par Stimuli corticaux sensoriels et que cette fonction est maintenue stable.

Current Status of Spinal Cord Monitoring

Spinal cord monitoring, as currently practiced, requires a source of sensory stimulus, transmitted through filters and amplifiers, to result in an enhanced signal, which is recorded and studied. Three techniques are under investigation. In one, the stimulation and responses are recorded directly from the dura. In another, the stimulus is applied to a peripheral nerve and the response is gathered from vertebral bone. The third utilizes responses in the form of cortical evoked potentials, with the stimulus applied to peripheral nerves.

Transoral microsurgical odontoid resection and spinal cord monitoring.

Four patients underwent transoral resection of the odontoid with utilization of microsurgical technique and spinal cord monitoring of somatosensory cortical evoked responses. All patients improved following surgery, and no operative morbidity or mortality was encountered. Spinal cord monitoring enhances the safety of the procedure and, reapplied at a later date, detects information of prognostic value.

Telemetering in vivo loads from nail plate implants

A system of multi-channel telemetry capable of broadcasting in vivo loading from orthopaedic implants is presented. Its use to monitor the two orthogonal bending moments about the nail plate junction of three implanted hip nails is described. Typical results from these cases in which the telemeterized implant was employed to treat proximal femoral fixation and osteotomy are given.

The Unstable Stable Thoracic Compression Fracture: Its Problems and the Use of Spinal Cord Monitoring in the Evaluation of Treatment

Two cases graphically demonstrate the well–established but often forgotten fact that not all compression fractures of the thoracic spine are stable. Adequate diagnosis depends on good roentgenograms and, if necessary, laminograms, to show whether the vertebral body is impacted or comminuted. Vertebral body fractures involving multiple, separated fragments are not stable, and inadequately tested cases can lead to either early or delayed increasing kyphosis, loss of alignment, and neurologic deficits such that operative stabilization (anterior, posterior, or combined) may be required. In 1 case of progressive deformity and neurologic loss which required surgery, somatosensory–evoked responses, which correlated well with clinical findings, were used to monitor spinal cord function.