Development of a versatile intra-articular pressure sensing array.

A new sensor array intended to accurately and directly measure spatial and time-dependent pressures within a highly curved biological intra-articular joint was developed and tested. To evaluate performance of the new sensor array for application within intra-articular joints generally, and specifically to fit within the relatively restrictive space of the lumbar spine facet joint, geometric constraints of length, width, thickness and sensor spatial resolution were evaluated. Additionally, the effects of sensor array curvature, frequency response, linearity, drift, hysteresis, repeatability, and total system cost were assessed. The new sensor array was approximately 0.6mm in thickness, scalable to below the nominal 12 mm wide by 15 high lumbar spine facet joint size, offered no inherent limitations on the number or spacing of the sensors with less than 1.7% cross talk with sensor immediately adjacent to one another. No difference was observed in sensor performance down to a radius of curvature of 7 mm and a 0.66±0.97% change in sensor sensitivity was observed at a radius of 5.5mm. The sensor array had less than 0.07 dB signal loss up to 5.5 Hz, linearity was 0.58±0.13% full scale (FS), drift was less than 0.2% FS at 250 s and less than 0.6% FS at 700 s, hysteresis was 0.78±0.18%. Repeatability was excellent with a coefficient of variation less than 2% at pressures between 0 and 1.000 MPa. Total system cost was relatively small as standard commercially available data acquisition systems could be utilized, with no specialized software, and individual sensors within an array can be replaced as needed. The new sensor array had small and scalable geometry and very acceptable intrinsic performance including minimal to no alteration in performance at physiologically relevant ranges of joint curvature.

Biomechanical analysis of compression screw fixation versus standard in situ pinning in slipped capital femoral epiphysis.

A slipped capital femoral epiphysis was created in 12 matched pairs of immature bovine femora using an anterior-to-posterior-directed shear force. All soft tissues, with the exception of the perichondrial ring, were removed before testing. One specimen from each pair was fixed with a single cannulated screw in standard fashion, whereas the contralateral specimen was fixed with a single screw that compressed the physis. The amount of compression achieved was quantified using Fuji film. Standard fixation yielded 1.4 MPa of pressure across the physis; compression fixation yielded 3.2 MPa, a 2.3-fold difference (p = 0.0001). The compression fixation was 47% more stiff than standard technique (p = 0.030), yet the differences in ultimate strength (p = 0.180) and energy absorbed at failure (p = 0.910) were not statistically significant. The stiffness of the compressed specimens remained less than that of the intact femora. Single-screw compression fixation of in vitro bovine femora was significantly more stiff than the current, widely used noncompression fixation technique, yet does not compromise the ultimate strength, energy absorbed, or the technical ease of single-implant fixation.

Electrosurgical methods for arthroscopic meniscectomy: A review of the literature.

PURPOSE: To outline the development of electrosurgical (radiofrequency) devices, explain the basic principles, and review the current orthopaedic literature regarding the application of electrosurgery to arthroscopic partial meniscectomy. MATERIALS AND METHODS: The history and principles of electrosurgery were obtained from various pertinent texts and journal articles. A literature search was performed using MEDLINE; reviewed articles consisted of articles in the English language cataloged between 1966 and January 1999. RESULTS: The history and principles of electrosurgery are reviewed. The articles pertaining to arthroscopic electrosurgical meniscectomy are discussed with a separate discussion on the potential complications of using radiofrequency energy for meniscal ablation, including articular cartilage damage, osteonecrosis, and damage caused by irrigant. CONCLUSION: Electrosurgery has been shown to be an effective tool in arthroscopic meniscectomy. Further research and refinement is warranted because it may show superiority to other methods in certain situations.

The effects of laser-induced collagen shortening on the biomechanical properties of the inferior glenohumeral ligament complex.

The purpose of this study was to determine the effects of laser-induced collagen shortening on the biomechanical properties of the inferior glenohumeral ligament complex. Fifty-seven bone-ligament-bone specimens underwent uniaxial tensioning to 10% strain. Approximately half of the specimens then underwent 10% shortening by lasing using a holmium:yttrium-aluminum-garnet laser. Both groups were again tensioned to 10% strain, and then loaded to failure. Ultimate strain and yield strain were significantly higher in the lased specimens than in the nonlased specimens. No significant difference was found for ultimate stress, yield stress, or elastic modulus between the two groups. Failure of the ligament did not appear to occur in the lased areas. The load-to-failure results suggested that the strength of the ligament complex was not significantly compromised by this lasing protocol.

Superior compressive strength of a calcaneal fracture construct augmented with remodelable cancellous bone cement.

Twenty-six paired, fresh-frozen cadaveric feet were disarticulated at the ankle joint, and the dome of the talus was potted. Stress-risers were placed along the medial, lateral, and posterior aspects of the calcaneus, and the specimen was loaded rapidly to failure in a testing machine to produce a type-IIB displaced intra-articular fracture according to the classification system of Sanders et al. One specimen of each pair was treated with standard internal fixation with bone-grafting (the control group), and the other was treated with similar fixation but with SRS (Skeletal Repair System) calcium phosphate bone cement placed in any osseous defect. All of the specimens were cured for twenty-four hours in a bath of saline solution at 37 degrees Celsius. The specimens were tested cyclically for ten cycles from zero to 100 newtons at one hertz and for 1010 cycles from zero to 350 newtons at one hertz. The deformation per cycle (millimeters per cycle), first-cycle deformation (millimeters), number of cycles to failure, and number of specimens withstanding the cyclical testing were calculated. The specimens were examined radiographically before and after fracture and after reconstruction and testing. A large difference in the results of the cyclical testing was noted. The specimens that had been augmented with the SRS bone cement had an average deformation of 0.00195 millimeter per cycle compared with 1.013 millimeters per cycle in the control group (p < 0.005). A similar magnitude of difference was noted when the results were stratified for good and poor-quality bone. Visual examination and radiographs demonstrated that a type-IIB displaced intra-articular fracture had been created reproducibly, and computed tomographic scans showed that nearly anatomical reconstruction had been achieved in all of the specimens. The computerized tomographic scans revealed good filling of the osseous voids and no evidence of failure of the cement after cyclical loading.

Biomechanical evaluation of polylactide absorbable screws used for syndesmosis injury repair.

Simulated syndesmosis injuries were created in 12 fresh-frozen, below-knee cadaver specimens. Six specimens were repaired with a 4.5 mm stainless steel screw, and six were repaired with a 4.5 mm polylactide screw. Three specimens of each group were tested in load to failure by axially loading with 1400 N and externally rotating to 90 degrees. Three specimens in each group underwent fatigue testing by axially loading with 700 N and applying 2.5 N-m of torque for 57,700 cycles. Radiographs and computed tomography scans were evaluated. None of the screws broke or failed. Similar load to failure was noted in polylactide and control groups. Fatigue testing revealed no significant change in stiffness. No significant screw damage was evident on radiographic or computed tomography evaluation. The data suggest that a polylactide screw has sufficient fatigue and failure strength to allow for healing of this injury in a clinical situation.

Mechanical response of the lumbar spine to seated postural loads.

STUDY DESIGN: In vitro force and deformation measurements formed the basis for determinate, quasistatic analysis of principal forces in the seated lumbar spine. OBJECTIVES: To explore the relationship between seated postures and the mechanical response in component tissues of lumbar intervertebral joints. SUMMARY OF BACKGROUND DATA: Despite the high prevalence of low back pain syndrome, the precise mechanisms relating specific mechanical loads to spinal degeneration are not well understood. Simultaneous, time-dependent measurement of anterior column forces and articular facet forces has not been presented previously. consequently, a determinate analysis of principal component forces has not been possible. METHODS: Twelve lumbar spines (L1-S1) were subjected to constant loading conditions while in flexed and extended seated postures. Time-dependent forces were measured in the anterior column at the L4 and L5 superior endplates and in the four facets of the L3-L4 and L4-L5 motion segments. A quasi-static analysis of sagittal plane forces was used to compute the remaining principal joint forces, including ligament, disc shear, and facet impingement forces. RESULTS: Component forces changed under static loading in both postures. There were significant differences between the mechanical responses of the two postures. Although the vertical creep displacement was greater in the extended seated posture (3.22 mm versus 2.11 mm), the escalation of forces was more severe in the flexed posture. CONCLUSIONS: The results suggest a mechanism of force balancing in lordotic postures under static loads, whereas flexed postures produce large increases to the tensile forces in the region of the posterior anulus.

Effect of partial versus complete plantar fasciotomy on the windlass mechanism.

Eight adult below-knee cadaver specimens were placed in a testing machine and loaded to 350 newtons according to a strict protocol. Arch height and length measurements were obtained in each specimen with the toes resting on the foot plate, dorsiflexed to 30 degrees, and maximally dorsiflexed manually. The plantar fascia was then divided from medial to lateral in one-quarter increments, and the effect on arch height and length measurements was assessed using the same loading protocol. A consistent decrease in the arch-supporting function on sequential sectioning of the plantar fascia was encountered. A less consistent decrease in the arch-supporting function was reflected by the increase in the height of the arch with sequential sectioning of the plantar fascia. The study demonstrates that partial plantar fasciotomy decreases the arch-supporting function of the plantar fascia in addition to weakening the structure. Strict surgical indications for this type of procedure should be maintained.

In vivo measurement of lumbar spinal creep in two seated postures using magnetic resonance imaging.

STUDY DESIGN: A magnetic resonance imaging technique was developed to measure creep in the lumbar spine in different seated postures. Owing to the fixed size and horizontal orientation constraints of the magnetic resonance imaging system bore, gravitational force was simulated in the horizontal plane while the subject assumed each of two near-seated postures: 1) flexed lumbar, and 2) extended lumbar. OBJECTIVES: One experimental subject was used to determine the feasibility of this technique designed to correlate spinal creep with different seated postures. SUMMARY OF BACKGROUND DATA: Although epidemiologic and pathologic studies have linked the seated posture with low back pain and disc degeneration, few in vivo studies have explored the mechanical response of the lumbar spine to seated postures. METHODS: Creep displacement of four lumbar discs (L3-S1) was measured from unloaded and loaded scans separated by 30 minutes of constant loading. Forty-eight images were measured from three trials in each posture, four sagittal slices per trial. RESULTS: Creep while in an extended posture (3.58 mm) was greater than creep while in a flexed posture (2.92 mm). System resolution was 0.78 mm. CONCLUSION: This technique can be used to discern between creep displacements in different postures.

Disruption of the ankle syndesmosis: biomechanical study of the ligamentous restraints.

We investigated the relative importance to stability of the four component ligaments of the distal tibiofibular syndesmosis. Eight fresh-frozen cadaver specimens were tested on a hydraulic test system during sequential cutting of the ligaments. The percentage resistance to 2 mm of diastasis was measured for the four ligaments. The anterior inferior tibiofibular ligament provided 35%, interosseous ligament 22%, superficial posterior inferior tibiofibular 9%, and deep posterior inferior tibiofibular 33%. These results have clinical implications with regard to injury. Damage to the syndesmosis should be assessed anteriorly and posteriorly at the time of examination. The interosseous ligament can be visualized arthroscopically.

Arthrodesis of the ankle. A comparison of two versus three screw fixation in a crossed configuration.

Sixteen human anatomic specimen ankles underwent an arthrodesis using two (medial and lateral) or three (additional anterior) screw fixation. It was shown that significantly greater compression was achieved with three screws (p = 0.001). The order of screw fixation was important--better compression was achieved by inserting the lateral screw first. The resistance to torque was measured on a materials testing system, and the three screw fixation technique was significantly better (p = .001). The recommended fixation is one screw medially, one laterally, and one anterior from the tibia to the talus.

Wear studies for development of an intervertebral disc prosthesis.

Two long-term wear behavior studies used in part of the process of selecting the best materials for a new all-metal orthopaedic spinal implant are presented. The relative and absolute wear volume rates of candidate metals were investigated through simulation of two bearing regions of an intervertebral disc prosthesis. It was determined gravimetrically that hot isostatically pressed Co-Cr-Mo alloy provided the lowest Co-Cr-Mo alloy wear debris generation. It had an average wear volume rate of 0.093 mm3/million cycles from a spring-in-pocket simulation and 0.126 mm3/million cycles from a hinge (pin-in-slot) simulation. The estimated total wear volume of a hot isostatically pressed intervertebral disc prosthesis with titanium-6%Al-4%V alloy springs was 2.9 mm3/million cycles.

A new transducer for facet force measurement in the lumbar spine: benchmark and in vitro test results.

A new transducer capable of direct measurement of time-dependent loads in human lumbar facet joints was developed and tested. The transducer was comprised of a force-sensitive resistor (FSR) in series with a pressure-sensitive film. A wide range of experiments revealed the performance attributes and limitations of the FSR. The output signal of the FSR is actually sensitive to both force and area of contact independently. Therefore, a pressure-sensitive film was used to quantify the contact area. At least two transformation equations were calculated for each FSR corresponding to known contact areas. Each equation was a linearization of the log of the FSR output vs the log of the applied ramp loads. Coefficients of determination (CD) were calculated for small (21 mm2) and large (32 mm2) contact areas, and were found to exceed 0.900 for all data. The average of nine cycles was nearly linear for some FSRs (CD of 0.999). FSR output signal and contact area were recorded in cadaveric lumbar facets under ramp load. The appropriate transformation equation was determined by a linear interpolation between benchmark equations based on the contact area measured in vitro. Facet force measurements compared well with those of other researchers. The transducer was found to be quite easy to use.

Design of an intervertebral disc prosthesis.

This article presents criteria for the design of a lumbar intervertebral disc prosthesis that take into consideration issues of endurance, materials behavior, geometry, kinetics, motion constraints, fixation to bone, and safety. The criteria and design philosophies discussed are generally applicable in the synthesis of any new implant system. Specifications of the design of a disc prosthesis are presented along with an evaluation of the design based on some of the preliminary test data.