Variability of forces applied by experienced therapists during spinal mobilization

OBJECTIVE: To determine the variation in forces used by different therapists during mobilization of the lumbar spine and the repeatability and reproducibility of individual therapists. DESIGN: An instrumented mobilization couch was developed to measure the forces applied to the trunk during spinal mobilization. BACKGROUND: Due to limitations in equipment design and data analysis, previous related studies demonstrate equivocal results. METHODS: The system was used to collect data from a sample of 30 experienced therapists to evaluate variation, repeatability and reproducibility during the application of five mobilization procedures. RESULTS: The variation in forces used by different therapists when performing the same technique was substantial, ranging between 63 and 347 N for one technique. During this procedure, 30% of the therapists were found to be relatively consistent, repeating the magnitude of the force applied at the first session within 5%. Others demonstrated considerable variation, exhibiting a difference as great as 34%. CONCLUSIONS: The inconsistency between experienced therapists has considerable implications for clinical practice. Changes in the magnitude and rate of loading are likely to have different effects due to the inherent viscoelastic behaviour of soft tissues. RELEVANCE: Spinal mobilization and manipulation techniques are frequently used by manual therapists in the treatment of musculoskeletal disorders. Despite the reliance on these techniques in clinical practice, there is little scientific evidence to substantiate their use. Before progress in this area can be made, it is necessary to characterize the forces used during typical mobilization procedures. The results can be used to develop teaching strategies and as a basis for comparative research on the efficacy of these techniques.

In vitro fatigue of human tendons

The purpose of this study was to determine the fatigue behaviour of human tendons in vitro. The testing was accomplished with the use of specially designed grips and the local measurement of tendon cross-sectional area. Ninety specimens prepared from Extensor digitorum longus (EDL) tendons of the foot were subjected to a cyclic square tension-tension stress waveform at physiological frequencies. The maximum tensile stress was normalised to values corresponding to prescribed levels between 10% and 90% of the calculated ultimate tensile strength (UTS) of 100 MPa. The minimum stress was set at 1% of the UTS. A replication of 10 specimens per stress level allowed the use of statistical models for the distribution of fatigue life. Results followed a linear model, of form S = 101.3 - 14.8 log(N), relating the normalised stress to the median number of cycles to failure, therefore suggesting the absence of an endurance limit. The Weibull distribution was found to describe adequately the probability of failure at each stress level. A model which takes into account in vivo healing was proposed. This model was able to explain the presence of intact tendons throughout the lifetime of an individual.

Compressive strains at physiological frequencies influence the metabolism of chondrocytes seeded in agarose

Articular cartilage is subjected to dynamic compressive loading that is known to influence chondrocyte metabolism. While the exact signalling mechanisms are unclear, it has been proposed that cell deformation plays a role and may stimulate a metabolic response through distinctive pathways. In this study, a well characterized model system in which chondrocytes are embedded in agarose was used to study the effect of dynamic cellular strain on three key metabolic processes, namely the synthesis of glycosaminoglycan, of DNA, and of total protein. Using a specially designed apparatus, 15% compressive strain amplitude was applied to agarose-chondrocyte cylinders statically or dynamically over a range of frequencies (0.3-3 Hz). Static and low-frequency strain (0.3 Hz) inhibited the synthesis of glycosaminoglycan, while a frequency of 1 Hz stimulated synthesis. Static strain reduced the level of thymidine uptake, whereas dynamic strain at all frequencies induced an increase in chondrocyte proliferation. Incorporation of tritiated proline was suppressed by all strain regimens tested. The three parameters investigated were each influenced by the dynamic strain regimens in a distinct manner, implying that the signalling mechanisms involved are uncoupled.

The development and characterization of an in vitro system to study strain-induced cell deformation in isolated chondrocytes

A model system has been developed to investigate cell deformation of chondrocytes in vitro. Chondrocytes were isolated from bovine articular cartilage by enzymatic digestion and seeded in agarose (type VII) at a final concentration of 2 x 10(6) cells.ml-1 in 3% agarose. Mechanical evaluation of the system showed no change in the tangent modulus of agarose/chondrocyte cultures over a 6-d culture period. The resulting agarose/chondrocyte cultures were subjected to compressive strains ranging from 5-20%. Cell shape was assessed by measuring the dimensions of the cell both perpendicular (x) and parallel (y) to the axis of compression and deformation indices (I = y/x) calculated. Cell deformation increased with the level of strain applied for freshly isolated chondrocytes. The cultures were maintained in medium that inhibits or stimulates matrix production (DMEM and DMEM + 20% FCS, respectively) in order to assess the effect of cartilaginous matrix on chondrocyte deformation. Matrix elaborated by the cells markedly influenced levels of cell deformation, an increase in matrix leading to a decrease in cell deformation. Freshly isolated deep zone chondrocytes were found to deform significantly more than surface zone chondrocytes, although this effect was lost after 6 d in culture. The elaborated matrix also altered the recovery characteristics of the chondrocytes following constant compressive strain of 15% for 24 h. Cells that had elaborated matrix took several hours to return to unloaded shape, while cells without matrix returned to the unloaded shape instantly.

Instrumentation of a mobilization couch for dynamic load measurement

Spinal mobilization or manipulation techniques are frequently used by physiotherapists in the treatment of musculoskeletal disorders. Many of these techniques involve the application of a varying force to the affected joint. Despite the routine use of these techniques, there are no objective measures to quantify their use. This paper describes the principles of the instrumentation of a mobilization couch to enable measurement of the forces applied during mobilization of the lumbar spine. The couch was linked to a personal computer with a 286 processor and data card for data collection. The system was found to be reliable and sensitive over the range of forces applied during mobilization. It has potential for use in many situations where measurements of forces applied to the patient are required.

Early progressive changes in tissue viability in the seated spinal cord injured subject

The patient with spinal cord injury is at high risk of tissue breakdown at all times due to a number of adverse factors, such as reduced mobility and anaesthesia. It is therefore essential that each patient is prescribed appropriate support media during initial rehabilitation. In this study, the effectiveness of prescribed wheelchair cushions has been assessed in terms of tissue response at the ischial tuberosities. A total of 42 subjects who had sustained traumatic spinal cord injury within 1 year were monitored on at least two occasions during initial rehabilitation. Changes in transcutaneous gas response (TcPO2 and TcPCO2) were monitored concurrently with regional interface pressures. A series of six transcutaneous gas variables were established, as markers of tissue viability. Non-parametric statistical analyses revealed some significant correlations between these variables. The results of this study also indicate that (1) spinal cord injury subjects with lesions below T6 show a progressive decrease in ability to maintain blood flow in sitting on prescribed support cushions and (2) SCI subjects with lesions above T6 show a progressive improvement in tissue viability status at the seating support interface. Therefore results imply that paraplegics are at a potentially higher risk of tissue breakdown than tetraplegics and thus require effective support cushions with strict adherence to a pressure relief regime.

The analysis of metabolites in human sweat: analytical methods and potential application to investigation of pressure ischaemia of soft tissues

A straightforward technique was developed for sweat collection applicable to tissues subjected to external load without introducing distortion of underlying tissues, and for analysis of six metabolites in the collected sweat. Chloride was measured colorimetrically and lactate, urea and urate by enzymatic methods on a centrifugal analyser. Sodium and potassium were measured by flame photometry. The methods showed good precision, recovery and linearity. To assess the technique sweat was collected: (i) from the sacrum, ischium, forearm and calf in healthy individuals at 32 degrees C for 1 h; (ii) from the sacrum of healthy subjects at ambient temperature for 9 h; (iii) at ambient temperature from the sacrum of a patient with a history of pressure sores. Sweat rates were greater at the sacrum and ischium than the calf or forearm. There were differences in the concentrations of lactate and urea between sites but these were smaller when expressed as amount secreted. Sweat rates were significantly lower in groups (ii) and (iii), but sweat could be collected reliably. This technique has potential clinical application to the investigation of susceptibility to pressure sores.

A biomechanical analysis of the plantar surface of soccer shoes

The incidence of severe injuries for soccer players may lead to long-term inactivity or, at worst, retirement from the game. Many of these injuries, particularly those involving the lower leg, can be attributed to adverse physical conditions at the interface between the soccer shoe and the playing support surface. This study investigated the biomechanical characteristics at this interface for a range of proprietary soccer shoes. An experimental system was designed and developed which, via a weighted pendulum arm making contact with a vertical column, provided controlled rotation of the forefoot of the soccer shoe on samples of playing surfaces. The overall rotation was found to depend on several physical and material factors. For example, the size 7 soccer shoes produced a statistically significant increase in axial rotation for the same impact energy compared to the larger sized shoes under test. In addition, flat soled shoes, designed for synthetic playing surfaces, produced consistently smaller rotations than shoes with either moulded or screw-in studs, although this finding depended on the moisture content of the playing surface. The pressure distribution within several soccer shoes was also measured using the F-Scan Gait Analysis System, for a subject walking across a grass surface. Results indicated differences in pressure distribution over the first metatarsal area of the foot, in existing shoe designs.

The effects of selective matrix degradation on the short-term compressive properties of adult human articular cartilage

The effects of proteoglycan and collagen digestion on the transient response of human articular cartilage when tested in unconfined compression were determined. Small cylindrical specimens of cartilage, isolated from the femoral head of the hip joint and from the femoral condyles of the knee joint, were subjected to a suddenly applied compressive load using a test apparatus designed to yield a transient oscillatory response. From this response values of the elastic stiffness and the viscous damping coefficient were determined. Cathepsin D and cathepsin B1 were used to digest the proteoglycan in some specimens, while in other specimens leukocyte elastase was used to attack the non-helical terminal regions of the Type II tropocollagen molecules and possibly the Type IX collagen molecule and thereby disturb the integrity of the collagen mesh. The results showed that proteoglycan digestion alone reduced the viscous damping coefficient but it did not significantly alter the elastic stiffness as determined from the oscillatory response. In contrast, the action of elastase reduced both the damping coefficient and the elastic stiffness of the cartilage. The results demonstrated the role of proteoglycans in regulating fluid transport in cartilage and hence controlling the time-dependent viscous properties. The elastic stiffness was shown to be dependent on the integrity of the collagen fibre network and not on the proteoglycans.

Transcutaneous gas tensions in the sacrum during the acute phase of spinal cord injury

The first few months following injury to the spinal cord requires constant care of the subject if tissue breakdown is to be avoided. The management of acute traumatic cases involves complete bedrest in a supine position with appropriately positioned pillows to minimize trauma to the bone prominences. This study assesses the effectiveness of the management procedure in terms of the tissue response at the sacrum of 15 acute spinal cord injured subjects. The measurement of mean interface pressures during a representative period of recumbency was performed and these were related to changes in transcutaneous gas tensions (TcPO2 and TcPCO2), which are reliable indices of tissue viability. A series of six variables was established which were compared to each other using non-parametric statistical analyses. It was shown that this group of subjects demonstrated a normal mechanism whereby the level of carbon dioxide was able to control the local vascular tone. The results also suggested that the practice of gapping at the sacrum should be revised to reduce mean sacral pressures and minimize the possibility of tissue breakdown, the risk of which is constant throughout the first three months following injury.

Biomechanical factors in wound healing following knee arthroplasty

A significant proportion of deep infections following knee arthroplasty are consequent on a failure of primary wound healing. The management of such infections is difficult and the functional outcome is poor. It is essential to ensure that the initial biomechanical conditions are such that wound healing is encouraged. The ability of a wound to heal is dependent on factors such as the orientation of the skin incision, the wound tension and the viability of the wound edges as assessed by skin oxygen tension estimations. Consideration of these factors has led us to advocate the use of the medial parapatellar incision for knee arthroplasty. Early, excessive knee flexion following arthroplasty can lead to significant wound edge hypoxia but by modifying the rehabilitation programme and with the administration of 24% oxygen to the patient in the perioperative period, this effect can be minimized. Modification of the surgical techniques involved in knee arthroplasty can help preserve the vascular supply and hence the viability of the wound edge, and thus achieve a suitable biomechanical environment for primary wound healing.

The recovery characteristics of soft tissues following repeated loading

Pressure relief at the patient support interface is important to avoid tissue breakdown by ischemia, particularly with debilitated subjects. However, there are still few guidelines to indicate the level of relief required for specific tissue areas. This paper examines the nature of the tissue recovery to repeated loading in compression. Loading was produced by (1) external application using an experimental system attached to the sacrum and (2) ischial support on a dynamic cushion. In both cases, the interface pressures applied for a prescribed time were related to changes in transcutaneous gas tension, the latter being an index of tissue viability. Results indicate two distinct responses to repeated loading. The normal response provides rapid and complete tissue recovery to unloaded values of transcutaneous oxygen tension. This was observed with all normal subjects and some of the debilitated subjects. There was also a group of debilitated subjects who demonstrated impaired and delayed tissue recovery. It is proposed that they are at most risk of developing tissue breakdown.

The biomechanical effectiveness of the Boston brace in the management of adolescent idiopathic scoliosis

The present study assesses the biomechanical effectiveness of the Boston brace on a group of 14 patients with adolescent scoliosis. Interface pressure measurements were performed at the appropriate compression pads within the brace when it was first applied and on two further sessions within the 6-month period of brace treatment. A corresponding series of Cobb angle measurements also were performed. Mean values of total force exerted through the brace of 58 +/- 18 N were associated with an initial curve correction of 37 +/- 21%. This mean force level was maintained throughout the study, although the mean curve correction at 6 months had reduced to 15 +/- 14%. In addition to biomechanical parameters, factors relating to the nature of the individual curve were important in determining the overall effectiveness of the Boston brace system.

Distraction physiolysis in the rabbit

Axial forces across the growth plate of the upper tibia during controlled distraction were studied in 24 rabbits close to skeletal maturity. Distraction rates of 0.13, 0.26, and 0.53 mm every 24 hours were applied through a dual-frame external fixator. Strain gauges were bonded to the fixator, and axial forces were estimated prior to and following distraction. The results demonstrate two distinct patterns. In one group, forces increased to maximum values of 20-32 newtons and then suddenly decreased on subsequent distraction. This force pattern indicated fracture of the growth plate with associated hyperplasia. In the other group, lower maximum forces of 6-18 newtons were produced at the end of the distraction period, which were associated with physeal hyperplasia without fracture. These results showed that response to controlled distraction was dependent upon both the level of force acting across the growth plate and the rate of distraction; hyperplasia was achieved with lower rates of distraction up to a critical peak force above which fracture occurred.

Biomechanical measurements of back shape and interface pressures in unsupported sitting

The paper describes the use of two non-invasive techniques to provide simultaneous measurements of interface pressures and back shape in the seated posture. A modified wheelchair was used to provide adjustments of various seating parameters. The effects of varying seat angulation with four normal subjects are reported. The lumbar lordosis, evident in the standing position, was absent in the sitting position. The changing of seat base angulation affected the balance between the main areas of seating support. For example, forward inclination of the seat reduced pr posterior thigh.

Changes in transcutaneous oxygen tension as a result of prolonged pressures at the sacrum

Prolonged pressures at the patient support interface can lead to the impairment of tissue viability resulting in tissue breakdown, particularly in debilitated individuals. However there are still few clinical guidelines to indicate safe levels of pressure and time for individual tissue areas. The effects of extended loading on transcutaneous oxygen tension (tcPO2) were examined at the sacrum of twenty debilitated subjects, prone to tissue breakdown. Each load was applied for a ten minute period through an indenter incorporating a commercial oxygen electrode. Loads were incremented until the tcPO2 levels were reduced below 2.7 kPa (20 mmHg), at which time the tissue was unloaded. The interface pressure for each applied load was measured. Results are presented in terms of relating applied pressure and time to relative changes in tcPO2. When all the results are combined it is clear that there is a range of applied pressures which significantly reduce the tcPO2 levels. The applied pressures to produce, for example, 50% reduction of the unloaded resting value, ranged from 3.0 kPa (22 mmHg) to 12.2 kPa (92 mmHg). This indicated the individual nature of the tissue response, which should be determined before clinical guidelines of safe pressure levels are established.

Effect of externally applied skin surface forces on tissue vasculature

The effect of surface forces on the morphologic pattern of skin capillaries was examined on the anterior surface of the forearm of healthy volunteers. A rig was designed to permit the application of uniaxial forces between two pads attached to the skin surface. A grid was printed onto the skin surface, and mechanical data are presented in terms of force intensity versus strain. During force application the skin capillaries were monitored using vital capillary microscopy. Mean values of force intensity and strain of 1.33N/mm and 10%, respectively, were sufficient to produce virtual obliteration of the blood flow to the capillaries. A simplified finite element model of the tissue was also employed to illustrate the effect of surface forces at different depths within the tissue. This study emphasizes the potentially damaging effects of surface forces on the integrity of the skin vasculature. If these forces are maintained or repeated, cell necrosis and eventual tissue damage would result.

Pressure distribution under the ischium of normal subjects

This paper describes the development of a system for measuring pressure distribution at the patient-support interface. The instrument uses pneumatic techniques under microcomputer control to measure pressures, with an overall accuracy of 3 per cent of full scale. The instrument has been used to investigate the pressure distribution under the ischium of four normal subjects, using a rectangular matrix. The study demonstrated a small variability in pressure distribution with a subject sitting still. Repositioning the subjects produced large variations in pressure readings, as a result of postural changes, which must be taken into account in the design of seating trials.

Substance P facilitation of memory: effects in an appetitively motivated learning task

Food deprived, heterogeneous strain (HS/IBG) mice were trained on two different discrimination tasks for food reinforcement. In one experiment animals were trained to make spatial discriminations in a T maze. Immediately after training they were given subcutaneous injections of either substance P (1 ng/g) or vehicle. Twenty-four hours later the animals were given reversal training in the same maze. The results showed that substance P-treated animals took significantly longer to acquire the reversal habit than did control mice. In a second experiment, animals were trained to make visual discriminations in a T maze. Immediately after reaching acquisition criterion animals were injected with either substance P (1 ng/g) or vehicle. Different groups of mice were retrained on the same task either 1, 2, 3, or 7 days after original learning. Savings scores were calculated and, at every interval, substance P-treated mice retained the task better than control animals. One interpretation of these data is that substance P-treated mice remembered the original task significantly better than vehicle-injected control animals.

The periosteum in growth plate failure

To investigate the periosteal contribution to the integrity of the growth plate, the authors subjected the proximal tibias of 110 male and female Sprague-Dawley rats between the ages of 25 and 65 days to axial and perpendicular forces to produce epiphysiolysis by use of a slow-loading technique and a cross-head speed of 10 mm per minute. Paired tibias of each rat were loaded under identical conditions, the only difference being the presence or absence of an intact periosteum. Mean values of body weight, tibial length, and cross-sectional areas of the growth plate after epiphysiolysis showed a linear increase with the age of the rat. No significant differences at the 5% level were observed in the tibial length and cross-sectional areas of the paired tibias. However, a consistently higher force was needed to produce epiphysiolysis with the periosteum intact. This was significant at all ages in both axial and perpendicular loading but was marked for young rats aged 45 days and under that were subjected to axial loading, where the level of significance was less than 1%. Calculated stress values showed a similar significant trend. The periosteum contributes significantly to the stiffness of the system, as shown by typical load deformation curves, and if it is divided, the gradient of the curve is clearly reduced up to epiphysiolysis. All periosteal fibers are not recruited in resisting a perpendicular force. Histologic examination of the specimens showed two patterns of failure corresponding to axial and perpendicular loading and also showed the failure path.

The effect of heel pads on the treatment of Achilles tendinitis: a double blind trial

Thirty-three subjects entered a blind-observer, random, prospective study of three forms of conservative treatment of sports-induced Achilles tendinitis, results being assessed by clinical and biomechanical parameters. Two patient groups received heel pads, ultrasound, and exercises, while the third received only ultrasound and exercises. All three groups showed some improvement at both 10 day and 2 month assessment, but the claimed benefit of viscoelastic pads widely used by athletes was not substantiated. The more striking benefit from ultrasound and exercises alone occurred in patients with a shorter history; a comparison of duration of injury in all three groups suggested this was an important factor influencing outcome. The study has highlighted the need for biomechanical outcome measures as well as for more objective clinical outcome measures in the assessment of physical therapy.

A study of uniaxial tension on the superficial dermal microvasculature

A spring-loaded apparatus was designed to apply uniaxial tension to forearm skin in 17 human subjects--10 normals, 6 psoriatics, and 1 patient with scleroderma. Simultaneously, the effects of stretching on the upper dermal vasculature were observed stereomicroscopically. Progressive changes (collapse) in the superficial microvasculature--vertical capillary loops and horizontal subpapillary plexus--with increasing tension were photographed. Force and strains were recorded at the points of disappearance of virtually all vessels. An average force of 11.9 newtons (N), accompanied by a mean strain of 10.3%, resulted in occlusion of all vessels. A much higher force (18.5 N) was necessary to occlude blood flow in the 1 patient with scleroderma. In summary, we have described a new technique for the study of mechanical forces on the blood supply of the epidermis. The data have shown that uniaxial tension has important effects on the superficial dermal microvasculature, resulting in impedance and obliteration of blood flow at relatively low magnitudes.

Mechanical characteristics of skin and underlying tissues in vivo

This paper is concerned with the mechanical characteristics of the complex composite of skin and subcutaneous tissues as they relate to the development of soft tissue replacements. Following a review of previous work and a summary of theoretical considerations, the results of a series of in vivo experiments aimed at quantifying the stiffness of skin and underlying tissues are reported. Variations in compressive properties with age, sex and body site are explained in terms of differences in anatomy and tissue structure.

Material properties of Velcro fastenings

An assessment of the material properties of three types of touch and close fasteners (Velcro) in general orthopaedic usage is presented. The materials were tested under various loading regimes using an Instron testing machine. The force-extension curves were analyzed and values determined for both the stiffness and strength of the various attachments. Particular reference was made to the alteration in attachment strength after cyclic loading. The strength of the standard Velcro was found to be least affected after cyclic loading to simulate continuous usage. A recommendation is made on the specific application of each type of Velcro based on their material properties.