A biochemical analysis of the squeeze test for sprains of the syndesmotic ligaments of the ankle

The "squeeze test" is a clinical test for detecting "stable" syndesmosis injuries. The test is positive when proximal compression of the calf produces pain in the area of the distal tibiofibular and interosseous ligaments. The purpose of this study was to examine what fibular movement, if any, occurs at the level of the ankle mortise when compression is applied proximally. A differential variable reluctance transducer was inserted into the origin and insertion of the anterior tibiofibular ligaments of fresh cadaver human lower limbs which had been prepared to simulate the clinical conditions of the squeeze test. Compression was applied both manually and with a specially designed clamp. Squeezing the calf caused separation of the distal fibula and tibia.

Angulated screw placement in the lateral condylar buttress plate for supracondylar femoral fractures

Certain supracondylar femoral fractures are not amenable to internal fixation with fixed angle devices. In these instances, the condylar buttress plate is the recommended alternative; however, this is a less rigid device. Because of the decreased rigidity and strength of this device, there is a tendency toward varus angulation and malunion. In six fresh-frozen human knee specimens, segmental osteotomies were created to mimic supracondylar femoral fractures. The medial cortex was completely removed to make the fracture unstable to varus deformity. The fracture was fixed with a lateral condylar buttress plate using 4.5 mm screws. Each specimen was tested once with all the screws installed perpendicular to the plate, and again with the middle screw, just proximal to the fracture, angled 45 degrees diagonally across the fracture into the subchondral bone of the medial femoral condyle. For the construct with all screws placed perpendicular to the buttress plate, the initial stiffness was 410 N/mm, and after 1000 cycles it was 230 N/mm. With a screw placed diagonally across the fracture site, stiffness increased to 833 N/mm on the first cycle, and 796 N/mm after 1000 cycles. In all specimens with the screws placed perpendicular to the plate, the distal fragment had a permanent varus deformity after 1000 cycles, under no load, of 0.91 mm. For the diagonal screw condition, the average magnitude for all six specimens was 0.42 mm. This simple means of screw angulation in the plate strengthened the overall construct to resist the tendency toward varus deformity. The attractive features include the ease of application, and the use of an existing construct.

Effect of variations in calcaneocuboid fusion technique on kinematics of the normal hindfoot

Calcaneocuboid fusion with lengthening of the lateral column of the foot has been advocated as a method of treating flatfoot deformity. This study was designed to determine how the length of the lateral column chosen or the position of the foot selected when performing this fusion affect hindfoot kinematics in normal cadaver feet. An electromagnetic tracking system was used to monitor the positions of the talus, calcaneus, navicular, and cuboid while the intact cadaver feet were moved passively and then under reproducible loads. Calcaneocuboid fusion was then performed on these feet first with the feet in neutral position and the lateral column of normal length, then lengthened 10 mm or shortened 5 mm, and then with the lateral column lengthened 10 mm and the feet positioned in plantar flexion and eversion or dorsiflexion and inversion. Kinematic measurements were made at each stage using the same loads. Fusing the calcaneocuboid joint with lengthening or shortening the lateral column and the feet in neutral position did not affect hindfoot joint motion compared with intact. Changing the position of the foot for fusion, however, resulted in significant decreases in motion in the talocalcaneal and talonavicular joints. Tibiotalar joint motion was unaffected. This study, therefore, demonstrates that when fusing the calcaneocuboid joint, attention should be paid to maintaining a neutral position of the foot.

The unstable iliac fracture: a biomechanical evaluation of internal fixation

Neither plating nor lag screw fixation of a displaced iliac wing fracture as part of an unstable pelvic ring disruption has been studied biomechanically. The purpose of this study was to compare the stability of various combinations of fixation, specifically contrasting lag screws placed between the tables of the ilium with plating in different locations across the fracture line. Various combinations of these fixation implants were evaluated for an unstable iliac fracture. A longitudinal iliac fracture was created in each of six hemi-pelvic specimens prior to testing. Compressive force, up to 500 N or to the magnitude necessary to displace the fracture 2 mm, was applied to the fracture line through the hemi-pelvis for each of the plate and lag screw combinations tested. There was no statistical difference between any of the implants or combinations tested. A single 3.5-mm reconstruction (cephalad) plate placed along the cephalad internal aspect of the iliac crest provided the least stability allowing 2 mm of displacement with a mean load of 80 N. The two combinations of fixation that required the greatest loads for 2 mm of displacement were a single 3.5-mm lag (cephalad) screw inserted into the iliac crest between the tables of the ilium coupled with either a 3.5-mm reconstruction (brim) plate placed along the internal aspect of the inferior iliac fossa at the pelvic brim (239 N) or a 4.5-mm lag (brim) screw between the inner and outer tables at the inferior aspect of the fracture just above the greater sciatic notch (225 N).

The role of fibular fixation in combined fractures of the tibia and fibula: a biomechanical investigation

OBJECTIVES

To determine whether adjunctive plating of the fibula with tibial fixation enhanced the stability of the construct under combined compressive and bending loads in simulated fractures of both the tibia and fibula.

METHODS

Each of twelve fresh cadaveric specimens (six pairs) with an intact knee, lower extremity, and foot was mounted on the table of a materials testing machine. An intramedullary (IM) rod locked in the distal femur allowed combined compression, and flexion, valgus bending, or varus bending loads to be transmitted from the actuator of the testing machine to the knee. Three displacement measurement transducers were mounted on the tibia at anterior, posterolateral, and posteromedial positions. Intact tibial deformations under load were measured. Then, in one specimen of each pair a 2 cm osteotomy was created near the tibial midshaft, which was stabilized with an external fixator. Tibial gap displacements were measured under the following conditions: (a) intact fibula, (b) osteotomized fibula, (c) fibula fixed with a plate, (d) fibula fixed with an Enders IM nail. In the other specimen of the pair, tibial fixation was performed with an interlocked unreamed IM nail, with the same successive stages of fibular fixation.

RESULTS

Osteotomy of the fibula significantly increased tibial defect motion when external fixation was used, and plating the fibula in this case significantly decreased motion. Using an Enders rod to stabilize the fibula instead of a plate, with tibial external fixation, produced smaller decreases in tibial defect site motion. With IM rod fixation of the tibia, osteotomizing the fibula had no effect on defect site motion or on its subsequent stabilization using a plate or IM rod.

CONCLUSION

Plating the fibula can decrease motion across a tibial defect, but only when less rigid (i.e., external) fixation is used.

Comparative biomechanical evaluation of different external fixation sidebars: stainless-steel tubes versus carbon fiber rods

Carbon fiber rods were developed to provide radiolucent sidebars for external fixation. In the present study, a single-plane, half-pin, double-bar external fixator construct with either stainless-steel tubes or carbon fiber rods was applied on the anteromedial surface of an osteotomized synthetic human tibia and evaluated for fixation rigidity. Testing was performed with the bone fragments in cortical contact and with a 5-mm midshaft gap between the fragments. The sidebars then were loaded to failure in bending. The results of this study show (a) that the carbon fiber rods were 15% stiffer than the stainless-steel tubes (p = 0.009) and (b) that the external fixator with carbon fiber rods achieved approximately 85% of the fixation stiffness of the external fixator with stainless-steel tubes. The loss of stiffness of the external fixator with carbon fiber rods is most likely due to the clamps being less effective in connecting the carbon fiber rods rigidly to the Schanz screws.

Cancellous bone screw thread design and holding power

This study was designed to isolate and evaluate the parameters of host density, outer diameter (OD), root diameter (RD), and pitch in cancellous bone screw design and their effect on holding power. Special emphasis was placed on screw pitch, which has been evaluated infrequently in the literature. Three groups of stainless steel V thread screws (group I, OD 4.5 mm, RD 3.0 mm; group II, OD 6.4 mm, RD 3.5 mm; group III, OD 6.4 mm, RD 4.2 mm) were machined with progressive increases in pitch from 12 to 32 threads per inch (TPI). Two densities of synthetic cancellous bone material (Pedilen, Ottobock, Minneapolis, MN, U.S.A.), 0.15 g/ml and 0.22 g/ml, were then prepared and molded into sheets 1.9 cm thick and the screw threads completely engaged. Push-out tests were performed using a servohydraulic testing machine (MTS, Minneapolis, MN, U.S.A.). Fifteen trials of each screw were tested in each material. The effect on holding power of the different parameters of the custom screws in order of importance was (a) host material density, (b) OD (c) pitch, and (d) RD. The groups with a 6.4-mm OD had a much greater holding power than did the group with a 4.5-mm OD (p < 0.001). A decrease in screw pitch (increased threads per inch) did itself have a significant improved effect on fixation for all groups in both pedilen densities (p < 0.001). In the two 6.4-mm screw groups studied, the difference in the two root diameters (4.2 mm vs. 3.5 mm) showed the smaller root diameter to give a greater holding power in the less dense 0.15 g/ml pedilen (p < 0.001). In the more dense 0.22 g/ml pedilen there was no difference (p = 0.26) between the root diameters. To optimize holding power, cancellous screws may be designed with a decreased pitch (increased TPI) over those commercially available today. Cannulated screws must have a larger cancellous thread root diameter to leave room for the central cannulation; this may decrease their holding power in less dense cancellous bone but not in denser bone.

Evaluation of new plate designs for symphysis pubis internal fixation

The purpose of this study was to evaluate and compare biplanar and other newly designed plates used for pubic symphysis internal fixation to other standard plates. Our data demonstrate that neither of the newly designed symphyseal plates, curved (Zimmer four-hole plate with either two or four 4.5-mm cortical screws) nor the biplanar (Zimmer six-hole plate with four 4.5-mm cortical screws in one plane and two 4.5-mm cortical screws in another) significantly reduce motion more than the other plates tested (Synthes two-hole 4.5-mm dynamic compression plate with two 6.5-mm cancellous screws and six-hole 3.5-mm reconstruction plate with four 3.5-mm cortical screws). All of the plate constructs were able to restore motion to nearly that of the intact symphysis pubis and sacroiliac joint. In summary, using the anteroposterior compression disruption pattern and model, the disrupted symphysis does gap under load but has no effect on sacroiliac joint gapping. It does permit increase in relative flexion angles between the wings of the pelvis. All of the plate systems tested restore normal gap motion at the symphysis and normal sacroiliac joint flexion.

Factors affecting the pullout strength of cancellous bone screws

Screws placed into cancellous bone in orthopedic surgical applications, such as fixation of fractures of the femoral neck or the lumbar spine, can be subjected to high loads. Screw pullout is a possibility, especially if low density osteoporotic bone is encountered. The overall goal of this study was to determine how screw thread geometry, tapping, and cannulation affect the holding power of screws in cancellous bone and determine whether current designs achieve maximum purchase strength. Twelve types of commercially available cannulated and noncannulated cancellous bone screws were tested for pullout strength in rigid unicellular polyurethane foams of apparent densities and shear strengths within the range reported for human cancellous bone. The experimentally derived pullout strength was compared to a predicted shear failure force of the internal threads formed in the polyurethane foam. Screws embedded in porous materials pullout by shearing the internal threads in the porous material. Experimental pullout force was highly correlated to the predicted shear failure force (slope = 1.05, R2 = 0.947) demonstrating that it is controlled by the major diameter of the screw, the length of engagement of the thread, the shear strength of the material into which the screw is embedded, and a thread shape factor (TSF) which accounts for screw thread depth and pitch. The average TSF for cannulated screws was 17 percent lower than that of noncannulated cancellous screws, and the pullout force was correspondingly less. Increasing the TSF, a result of decreasing thread pitch or increasing thread depth, increases screw purchase strength in porous materials. Tapping was found to reduce pullout force by an average of 8 percent compared with nontapped holes (p = 0.0001). Tapping in porous materials decreases screw pullout strength because the removal of material by the tap enlarges hole volume by an average of 27 percent, in effect decreasing the depth and shear area of the internal threads in the porous material.

Internal fixation for the transforaminal sacral fracture

The mechanical stability of alternate forms of internal fixation of the transforaminal sacral fracture were compared. A transforaminal sacral fracture was made in each of 6 fresh-frozen cadaveric pelvic specimens. Implants compared for fixation included: a single and 2 fully threaded iliosacral screws inserted through the posterior ilium and anchored into the first sacral vertebral body both with and without the addition of a posterior tension band plate; and 2 transiliac bars inserted through the posterior tubercles. The femora of each specimen were potted and fixed to the table of a materials tester. The pelvis was restrained only from flexing and extending, and a compressive load was applied through the lumbar spine, representing a standing loading condition. Flexion of the sacrum and displacement at the fracture site were measured during loading. Although creation of the injury increased motion considerably, there was no measurable increase in stability provided by any of the implants or combination of implants in this model when an anatomic reduction was obtained.

A comparative biomechanical evaluation of a noncontacting plate and currently used devices for tibial fixation

The appearance of porous bone under fixation plates during fracture healing, attributed to disturbance of blood supply by the plate, has led to new plate designs with reduced plate to bone contact. The fixation stability afforded by these devices, in comparison to implants commonly used for fixation, is not well known. Therefore, the construct stiffnesses of osteotomized synthetic tibiae fixed with dynamic compression plates, external fixators, or two configurations of noncontact plates were compared in axial compression, bending, and torsion with and without cortical contact at the osteotomy site. The results of this study show that (1) the noncontact plated constructs achieve comparable fixation rigidity to constructs using dynamic compression plates or external fixators, if applied at a distance no greater than 5 mm from the surface of the tibia, and (2) the fixation rigidity of the noncontact plate decreases as the distance between plate and bone surface increases.

Comparison of the effect of reamed and unreamed locked intramedullary nailing on blood flow in the callus and strength of union following fracture of the sheep tibia

This study was performed to compare the effects of reamed and unreamed locked intramedullary nailing on blood flow in the callus and early strength of union in a fractured sheep tibia model. After the creation of a standardized short spiral fracture by three-point bending with torsion, each tibia was stabilized by the insertion of a locked intramedullary nail. Ten animals were allocated randomly into two groups: one that had reaming prior to nail insertion and one that did not. Blood flow was measured in real time with use of laser Doppler flowmetry. Endosteal perfusion was determined at the fracture site before and after nail insertion. Perfusion of the callus was measured at three locations (proximal diaphysis, fracture site, and distal diaphysis) and at three time intervals (2, 6, and 12 week follow-up). All animals were killed 12 weeks postoperatively, and the tibiae were tested to failure in four-point bending. Nailing with reaming resulted in a larger decrease in overall endosteal perfusion than nailing without reaming (p < 0.015). The presence or absence of reaming did not affect blood flow within fracture callus. Perfusion of callus was greatest at 6 weeks of follow-up. Bending strength and stiffness were the same in both groups at 12 weeks. The study demonstrated that perfusion of callus and early strength of union are similar following intramedullary nailing with or without reaming.

The acetabular T-type fracture. A biomechanical evaluation of internal fixation

The purpose of this study was to evaluate the stability of different types of internal fixation requiring anterior, posterior, or combined surgical approaches for the T-type acetabular fracture. Eight specimens were loaded 25 times in a cyclic manner to 150 N for each type of fixation construct evaluated. The model attempted to range the hip through an arc of motion anticipated in the limited activities expected after surgery. Strain gauges were placed on the 3 fracture planes of the T-type fracture. These gauges measured displacements. Internal fixation consisted of a single anterior column plate, compared with a single posterior column plate, compared with simultaneous placement of both anterior column and posterior column plates. The differences in displacements measured between the 3 types of fixation at each of the 3 fracture sites were not statistically significant. In evaluating motion at the anterior fracture line, the anterior plate made the largest contribution to stability, although this result was not statistically significant. The posterior plate similarly contributed most to posterior fracture stability, although again not statistically significant. Each of the 3 fixations controlled the inferior fracture line motions in a comparable manner.

Anterior versus posterior provisional fixation in the unstable pelvis. A biomechanical comparison

Pelvic ring injuries with associated hemorrhage often require provisional fixation to achieve tamponade. Biomechanics information regarding these provisional fixators is unknown. Six fresh-frozen cadaveric pelvic specimens were physiologically loaded, first intact and then after each of the following modifications: disrupted--unilateral superior and inferior rami osteotomies, ipsilateral anterior and posterior sacroiliac joint, and sacrospinous and sacrotuberous ligament disruption; disrupted and with placement of a Ganz pelvic resuscitation clamp; and disrupted and with placement of a simple anterior 2-bar external fixator. This injury resulted in significant motion at the disrupted rami and the injured sacroiliac joint, compared with the intact pelvic specimen. Motions at the superior ramus and injured sacroiliac joints were significantly (p < 0.05) greater than the intact specimen, with both the external fixator and the Ganz clamp. Motions at the superior ramus and injured sacroiliac joints were not significantly (p < 0.05) different when comparing the Ganz clamp to the external fixator. However, the anterior external fixator decreased motion to a greater degree at the disrupted rami, whereas the Ganz clamp decreased motion to a greater degree at the disrupted sacroiliac joint.

Biomechanical simulation of the anteroposterior compression injury of the pelvis. An understanding of instability and fixation

Seven fresh cadaveric pelvic specimens were biomechanically analyzed. Testing was first performed on intact pelves and then after progressive disruption of the (1) symphysis pubis, (2) unilateral anterior and interosseous sacroiliac ligaments and capsule, (3) ipsilateral sacrospinous and sacrotuberous ligaments; and fixation with a 4.5-mm narrow dynamic compression plate at the symphysis pubis, or a 4.5-mm narrow dynamic compression plate at the anterior sacroiliac joint with and without the symphysis pubis plate, or a 7.0-mm sacroiliac lag screw anchored into the S1 vertebral body with and without the symphysis pubis plate. Symphyseal gapping occurred after isolated symphysis pubis disruption. With additional disruption of the unilateral sacroiliac joint ligaments, symphysis pubis displacement was unaffected; however, the injured sacroiliac joint gap displacement, and sacroiliac joint flexion angulation on both intact and injured sides increased as compared to the specimen in the intact state. Further disruption of the ipsilateral sacrotuberous and sacrospinous ligament complex produced little additional motion at either symphysis pubis or sacroiliac joints. Plate fixation of the symphysis pubis alone reduced symphysis pubis motion, but not sacroiliac motion. Use of sacroiliac fixation alone without a symphysis pubis plate did not affect symphysis pubis motion. The symphysis pubis plate is the key to stabilizing symphysis pubis motion, and similarly, sacroiliac joint fixation is required to control sacroiliac joint motion. Both single iliosacral screws and plates produced equivalent decreases in sacroiliac joint motion.

Internal fixation of the unstable anterior pelvic ring: a biomechanical comparison of standard plating techniques and the retrograde medullary superior pubic ramus screw

The purpose of this study was to evaluate pubic ramus fracture fixation. This biomechanical evaluation compared standard plating techniques with retrograde medullary screw fixation of a superior pubic ramus fracture in a pelvic fracture model. Six fresh-frozen, cadaveric pelvic specimens with a mean age of 79 years were harvested. These specimens were physiologically loaded according to the following modifications and instrumentations: (a) intact; (b) an APC-II unstable pelvic injury, specifically, unilateral superior and inferior rami osteotomies combined with ipsilateral anterior sacroiliac (SI) joint, sacrospinous, and sacrotuberous ligamentous disruptions, without fixation; (c) disrupted as in (b) but fixed anteriorly with a 10-hole 3.5-mm reconstruction plate contoured to the superior ramus and secured with four 3.5-mm cortical screws; (d) disrupted as in (b) but fixed anteriorly with a 10-hole 3.5-mm reconstruction plate contoured to the superior ramus and secured with six 3.5-mm cortical screws; (e) disrupted as in (b) but fixed anteriorly with a 4.5-mm retrograde medullary superior pubic ramus cortical screw 80 mm long (medial to the hip joint); and (f) disrupted as in (b) but fixed anteriorly with a 4.5-mm retrograde medullary superior pubic ramus cortical screw 130 mm long that was extraarticular and engaged the lateral iliac cortex cephalad to the ipsilateral hip joint. The posterior disruptions of the pelvic ring were not fixed. The APC-II injury created in this study resulted in significant (p < 0.05) motion at the disrupted rami and the injured SI joint, compared with the intact pelvic specimen.(ABSTRACT TRUNCATED AT 250 WORDS)

Box plate fixation of the symphysis pubis: biomechanical evaluation of a new technique

The purpose of this study was to compare common techniques of pubic symphyseal fixation with a new method, the "box plate," for fractures of the pelvis where the bone is osteopenic. This symphyseal fixation construct consists of two, two-hole, 4.5-mm narrow dynamic compression plates (DCP) oriented parallel to one another. One plate is recessed within the symphysis, and the other is located on the pubic tubercles. The plates are interlocked using two 6.5-mm fully threaded screws, forming a box-like construct. To determine the mechanical properties of this construct, five fresh, cadaveric pelvic specimens with a mean age of 75 years were harvested. The femora of each specimen were potted into containers and fixed to the base of a materials testing machine. The pelvis was constrained from rotating about the hip joints by anterior and posterior restraints. A vertical compressive load was applied through the lumbar spine. Force to a magnitude of 1,000 N was applied through three cycles. Gapping motions at the symphysis pubis (SP) and the sacroiliac (SI) joints, and flexion-extension of the sacrum with respect to the ilia were measured under the following conditions: (a) intact, (b) SP ligament, unilateral anterior SI ligaments, and ipsilateral sacrospinous and sacrotuberous ligaments disrupted (anteroposterior compression type II injury), and these injuries fixed using (c) a 4.5-mm narrow two-hole DC plate placed on the superior SP held by two cancellous bone screws, (d) the DC plate well as a single 7.0-mm cannulated cancellouoffliosacral lag screw across the injured SI joint, (e) the DC plate and a five-hole 3.5-mm reconstruction plate on the anterior SP, (f) a 3.5-mm, four-hole, DC plate on the superior SP using four fully threaded screws, and (g) the box plate symphyseal construct described above. All fixations reduced SP joint gapping compared to the disrupted joint. However, all but the box plate still allowed significantly greater motion than the intact SP joint. No fixation significantly reduced SI joint gapping or sacral flexion compared to the injured state.

Extramedullary skeletal traction for intramedullary femoral nailing

The through-and-through forms of femoral skeletal traction that are often used during intramedullary nailing for femur fractures can present two problems: (a) impingement with the intramedullary nail, requiring repositioning of the traction pin intraoperatively under radiographic control, to a position that may not optimally control the fracture; and (b) the risk of contaminating the intramedullary canal that will soon contain the metallic fixation device, especially when placed in the emergency room or at the bedside. Two new forms of extramedullary skeletal femoral traction are presented. The pullout strength and optimal positioning of these devices on the distal femur were evaluated. The first form of extramedullary traction evaluated was the large AO/ASIF pinless clamp. The second form of extramedullary traction, the extramedullary skeletal clamp, was based on a modification of the Gardner-Wells tong. These two forms of skeletal traction were compared with standard tensioned Kirschner wire (K-wire) through-and-through traction. Six fresh-frozen distal femora from donors (average age 29 years) were used to test the three femoral traction devices. Five of these specimens were skeletally mature. A total of 38 pullout tests were conducted. The pullout strength of the tested devices was in the following descending order: (a) tensioned K-wire; (b) extramedullary skeletal clamp (in the optimal metadiaphyseal position, 77% the strength of the K-wire); and (c) large ASIF pinless clamp (in the optimal metaphyseal position, 46% the strength of the K-wire). The mode of failure for each device was cut out through or from the bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical effects of internal fixation of the distal tibiofibular syndesmotic joint: comparison of two fixation techniques

This biomechanical study compares two methods of internal fixation of the tibiofibular syndesmosis used in Weber type C malleolar fractures of the ankle. The transverse syndesmotic 3.5-mm screw was compared with two 1.5-mm Kirschner wires introduced obliquely across the distal tibiofibular syndesmosis. The influence of implants on distal tibiofibular joint motion and contact characteristics of the intact ankle joint were determined. Up to 1.25 mm of lateral displacement and 2 degrees of external rotation of the lateral malleolus during uninjured ankle dorsiflexion was recorded. Both techniques stabilized the injured syndesmotic joint and limited its normal motion during flexion and extension of the ankle. Pressure distribution displaced laterally in internally stabilized ankles compared with intact specimens, regardless of the type of fixation used. Therefore, both techniques alter joint biomechanics equivalently compared with the intact ankle.

Effect of sterilization and storage treatments on screw pullout strength in human allograft bone

Many conflicting studies have been performed evaluating the effects of sterilization and storage treatments on the mechanical properties of allograft bone. In the current study, four right and left matched, sterile, deep-frozen, tibial pairs from human donors with an average age of 32 years were tested. One tibia from each pair served as a matched control for the opposite side. Each tibia was cut into four equal segments. One segment of each tibia underwent no treatment; the other three underwent one of the following treatments: irradiation, freeze drying, or ethylene oxide (ETO). Screw pullout tests were performed using four 3.5-mm cortical screws per segment. Sixteen screw pullout tests were performed for each allograft treatment. The freeze-dried specimens required significantly less force for screw pullout. The screw pullout force for the irradiated specimens and the ETO specimens did not significantly differ from their controls. These results indicate that freeze drying affected screw pullout strength. This method of processing should be questioned for structural allografts in which screw fixation is mandatory. The use of irradiation or ETO for sterilization may not have an adverse effect on screw pullout strength.

Biomechanics of indirect reduction of bone retropulsed into the spinal canal in vertebral fracture

The biomechanics of indirect reduction of bone fragments retropulsed into the spinal canal in a burst fracture were investigated. In this model, tunnels were created in vertebrae L1 and C5 oriented anterior-to-posterior, allowing access to the posterior longitudinal ligament. A probe containing a load-sensing tip was passed through the tunnel. Both the location of the tip and the load acting on it by posterior deflection of the posterior longitudinal ligament were measured. In the lumbar spine, distraction was applied by spinal instrumentation that also permitted independent kyphotic-lordotic alignment of the vertebrae. In the cervical spine, axial traction was applied through direct loading. Several clinically relevant observations were made. It was not possible to produce an anteriorly directed force in the posterior longitudinal ligament at less than 35% canal occlusion, partly because the posterior longitudinal ligament stands away from the midbody of the vertebra. Distractive forces of up to 150 N were applied in the lumbar spine, which were nearly equal to the tensile breaking strength of the isolated posterior longitudinal ligament. Regardless of the relative sagittal plane angulation of the vertebrae, distraction was the governing factor in generating force in the posterior longitudinal ligament. Because positioning the vertebrae in lordosis before applying distraction significantly slackens the posterior longitudinal ligament, it is suggested that distraction be applied before angular positioning of the vertebrae is performed.

Contact characteristics of the subtalar joint: load distribution between the anterior and posterior facets

The pressure distribution properties of the normal talocalcaneal joint were studied in 13 fresh cadaver specimens using pressure-sensitive film. The film was inserted into the joint through a posterior approach for the posterior facet and an anteromedial approach for the anterior/middle facet. Specimens, comprising the distal half of the tibia and fibula and the intact ankle and foot, were positioned in neutral in the sagittal plane. In the coronal plane, specimens were positioned in neutral, inversion, or eversion, and the contact characteristics were determined in varying positions, with and without loading the fibula, under axial loads of 350 N, 700 N, 1,050 N, and 1,400 N. The transducers were video imaged for quantitative analysis of area and pressure. The contact/joint area ratio increased significantly with applied load in the posterior facet [e.g., in eversion from 0.336 (SD = 0.195) at 350 N to 0.631 (SD = 0.225) at 700 N], as did the proportion of the contact area greater than 6 MPa, indicating an increase in contact pressure. The contact/joint area ratio was significantly lower in inversion than in any other position of the foot; however, high-pressure zones were similar, indicating that higher pressures occur in inversion. In the anterior/middle facets both contact/joint area ratio and high pressure/contact area ratios increased significantly to 700 N, but not with further load increase. At 350 N the anterior/middle facet contact area was 31% that of the posterior facet, yet it carried 63.3% of the load of the posterior facet, so its mean contact pressure was 1.44 MPa compared with 0.93 MPa for the posterior facet.(ABSTRACT TRUNCATED AT 250 WORDS)

Contact characteristics of the subtalar joint: the effect of talar neck misalignment

In this study we determined the effects of misalignment of the talar neck on the contact characteristics of the subtalar joint. Each of seven fresh cadaver lower extremities was mounted in a loading jig and a vertical load was applied, 90% of which was directed through the tibia and 10% through the fibula. The foot was allowed to displace freely in the horizontal plane so that relative rotations, known to occur in the subtalar joint, would not be prevented. Pressure-sensitive film, inserted into the posterior and anterior/middle articulations, was used to quantify changes in contact characteristics. After testing in the normal condition, the talar neck was osteotomized and stabilized with internal and external skeletal fixation. Contact characteristics were then determined in each of the following stages: anatomic realignment, or with 2-mm displacement of the talar neck either dorsally, medially, laterally, or complex (dorsal and varus) with respect to the body of the talus. Measurements showed no significant changes in overall contact area or high pressure area in the posterior facet, although four of the seven specimens demonstrated increased localization of the contact area into two discrete regions. The combined anterior/middle facet, on the other hand, was significantly unloaded by all but medial displacement of the talar neck. An extraarticular load path and/or increased loading directly on the talonavicular joint was presumed to account for the loss of load transfer in the talocalcaneal joint.

Parathyroid hormone decreases HCO3 reabsorption in the rat proximal tubule by stimulating phosphatidylinositol metabolism and inhibiting base exit

The mechanism of inhibition of HCO3 transport by parathyroid hormone (PTH) in the proximal tubule is not clearly defined. Previous studies in vitro have suggested that this effect is mediated via cAMP generation, which acts to inhibit Na/H exchange, resulting in cell acidification. To examine this question in vivo, intracellular pH (pHi) was measured in the superficial proximal tubule of the rat using the pH-sensitive fluoroprobes 4-methylumbelliferone (4MU) and 2',7'-bis(carboxyethyl)-(5, and 6)-carboxyfluorescein (BCECF). PTH was found to alkalinize the cell. This alkalinization suggested inhibition of basolateral base exit, which was confirmed by in situ microperfusion studies: lowering HCO3 in peritubular capillaries acidified the cell, an effect blunted by PTH. Removal of luminal Na promoted basolateral base entry, alkalinizing the cell. This response was also blunted by PTH. Readdition of luminal Na stimulated the luminal Na/H exchanger, causing an alkalinization overshoot that was partially inhibited by PTH. cAMP inhibited luminal H secretion but did not alkalinize the cell. Stimulation of phosphatidylinositol-bis-phosphate turnover by PTH was suggested by the effect to the hormone to increase cell Ca. Blocking the PTH-induced rise in cell Ca blunted the effect of the hormone to alkalinize the cell, as did inhibition of phosphatidylinositol breakdown. Furthermore, stimulation of protein kinase C by a phorbol ester and a diacylglycerol applied basolaterally alkalinized the cell and inhibited luminal H secretion. The findings indicate that both arms of the phosphatidylinositol-bis-phosphate cascade play a role in mediating the effect of PTH on the cell pH. The results are consistent with the view that PTH inhibits base exit in the proximal tubule by activation of the phosphatidylinositol cascade. The resulting alkalinization may contribute, with cAMP, to inhibit apical Na/H exchange and the PTH-induced depression of proximal HCO3 reabsorption.

Force applied during tracheal intubation

This study quantitated the force applied during tracheal intubation to determine (a) whether the force differed among novice and experienced intubators, and (b) whether the force required differed when intubating patients' tracheas versus intubating the trachea of a commonly used training mannequin. We studied 27 tracheal intubations performed by 17 experienced (greater than 100 prior intubations) and 10 novice (less than 10 prior intubations) intubators. Each intubation was performed with a No. 3 Macintosh blade instrumented with strain gauges to determine force applied in the sagittal plane. The mean force applied was 24.6 +/- 2.9 N (mean +/- SEM) and the maximum force applied by each intubator was 47.6 +/- 3.8 N. There was no difference between groups in the mean force applied (28.8 +/- 6.1 N for the novice group vs 22.3 +/- 2.7 N for the experienced group, P = 0.27) nor in the maximum force applied (55.1 +/- 6.5 N for the novice group vs 43.2 +/- 4.7 N for the experienced group, P = 0.08). The only difference was in the impulse (force x duration), which was more for the novice group largely because of the longer average duration of intubation (40 +/- 12 s vs 19 +/- 4 s, P = 0.06). Among experienced intubators, we found that applied force correlated with patient weight and Mallampati class. Intubation of the Laerdal Airway Management Trainer required mean forces comparable to those required in patients (26.6 +/- 2.5 N vs 22.3 +/- 2.9 N), although the maximum force applied during the intubation effort was greater (58.3 +/- 4.7 N vs 43.2 +/- 4.7 N, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of age and peripheral vascular disease on the circulatory and mechanical response of skin to loading

The skin and subcutaneous soft tissues of amputation residual limbs are required to withstand externally applied loads of greater magnitude than similar tissues of the intact lower limb. Increased age and poor circulatory status may contribute to the increased risk of tissue injury seen in this population. This study evaluates the effects of age and circulatory status as risk factors for skin injury resulting from externally applied forces. Twelve young control (YC), six elderly control (OC) and 11 subjects with peripheral vascular disease (PVD) were studied. After base-line ankle arm index (AAI) measurements, TcPO2 electrodes were applied 10 cm below the knee over the medial surface of the tibia and the muscle belly of tibialis anterior. TcPO2 measurements and tissue displacements were obtained under the influence of incremented, normally oriented, external loads. The sensitivity of the tissues to applied loads was determined by calculating the load at which the TcPO2 reached zero. The stiffness of the tissues (displacement/load) was calculated under high (greater than 40 mm Hg) and low (less than 20 mm Hg) loading conditions. No difference was noted in tissue sensitivity to applied loads between the OC and YC populations. The TcPO2 decreased to zero in the PVD population at significantly lower applied loads than both the OC and YC populations. The tissue stiffness of the PVD and the OC populations over bone was greater than the YC population, but no significant differences were noted between the PVD and the OC populations. In summary, increased age does not result in a greater tissue sensitivity to externally applied loads, in spite of the demonstrated increased tissue stiffness.(ABSTRACT TRUNCATED AT 250 WORDS)

Circulatory and mechanical response of skin to loading

We investigated the tolerance of skin to mechanical loading over the tibia and over the tibialis anterior muscle in 12 normal volunteers. Surface load, subcutaneous tissue pressure, skin deformation under load, and transcutaneous partial pressure of oxygen (TcPO2) were simultaneously monitored. The skin over bone showed a significantly stiffer load deformation relationship than the skin over muscle (p less than 0.001). The displacement required to reduce TcPO2 to 0 over bone, 1.1 +/- 0.3 mm (mean +/- standard deviation), was significantly less than that required over tibialis anterior muscle, 5.4 +/- 1.1 mm (p less than 0.001). The applied pressure required to reduce TcPO2 to 0 was significantly greater for skin over muscle (71 +/- 16 mm Hg) than for skin over bone (42 +/- 8 mm Hg) (p less than 0.001). However, the subcutaneous pressure required to reduce TcPO2 to 0 was not significantly different for skin over muscle (36 +/- 11 mm Hg) than for skin over bone (28 +/- 10 mm Hg) (p greater than 0.05). Our results indicate that skin over muscle tolerates greater locally applied loads and deformations because the pressure is lower within the tissue than when similar loads and deformation are applied to skin over bone. Cutaneous perfusion, as indicated by TcPO2, seems to be linked more closely to the subcutaneous tissue pressure than to the surface load or deformations. These results provide some data for predicting mechanical and physiologic response to locally applied loads such as those that may be encountered in prosthetic wear.

Transcutaneous oxygen tension as a predictor of success after an amputation

We measured local transcutaneous oxygen tension at the foot and proximal and distal to the knee in 162 patients who then had 206 amputations. When the values for oxygen tension at the foot and distal to the knee were compared with the success or failure of healing after an amputation of the foot or distal to the knee, respectively, a clearly increasing probability of failure was correlated with decreasing transcutaneous oxygen tension. However, even at a tension of zero the probability of failure was not 100 per cent. The results were similar for diabetic and non-diabetic patients. Preoperative values for transcutaneous oxygen tension were a much more consistent predictor of success or failure of healing after an amputation of the foot or distal to the knee than were measurements of systolic blood pressure at the ankle, but neither was predictive of the outcome after an above-the-knee amputation.

Axial heterogeneity of intracellular pH in rat proximal convoluted tubule

In the proximal convoluted tubule (PT), the HCO3- reabsorptive rate is higher in early (EPS) compared with late proximal segments (LPS). To examine the mechanism of this HCO3- reabsorption profile, intracellular pH (pHi) was measured along the superficial PT of the rat under free-flow and stationary microperfusion using the pH-sensitive fluorescence of 4-methylumbelliferone (4MU). With 4MU superfusion, pHi was found to decline along the PT. Observation with 365-nm excitation revealed that EPS were brightly fluorescent and always emerged away from their star vessel. Midproximal segments were darker and closer to the star vessel which was surrounded by the darkest LPS. Decreasing luminal HCO3- from 15 to 0 mM lowered pHi in both EPS and LPS, but pHi remained more alkaline in EPS with both perfusates. Thus the axial decline in pHi along the PT is due to both luminal factors and intrinsic differences in luminal H+ extrusion in PT cells.

Relationship between transcutaneous oxygen tension, ankle blood pressure, and clinical outcome of vascular surgery in diabetic and nondiabetic patients

We measured ankle systolic blood pressure (ABP) and limb transcutaneous oxygen tension (TcPO2) before and after 53 vascular procedures performed to relieve limb-threatening ischemia. We compared changes in ABP and TcPO2 and also compared these measurements of limb hemodynamics with the clinical outcome of the vascular procedures. For the procedures performed on patients without diabetes, both ABP and TcPO2 registered similar changes after surgery. Furthermore, those nondiabetic patients who had a postoperative ABP greater than 75 mm Hg or TcPO2 greater than 20 mm Hg showed resolution of the clinical symptoms within 60 days after surgery. All patients falling below these levels underwent a subsequent limb amputation. The results differed somewhat for procedures performed on patients with diabetes. First, a number of diabetic patients showed high ABP in conjunction with low TcPO2. We attribute these observations to the high incidence in diabetic patients of calcific medial stenosis leading to artificially elevated ABP measurements. Second, the clinical outcome among diabetic patients was uncorrelated with the postoperative ABP and was poorly correlated with postoperative TcPO2. Those diabetic patients with postoperative TcPO2 below 20 mm Hg showed unfavorable clinical outcomes, but many patients with postoperative TcPO2 greater than 20 mm Hg and postoperative ABP greater than 75 mm Hg also showed unfavorable clinical outcome (slow healing of ulcers, persistence of rest pain, and/or an amputation on the limb). These data suggest that among our patients with diabetes, simple relief of limb ischemia was not sufficient to result in a trouble-free clinical course. We conclude that TcPO2 is a useful replacement or adjunct to ABP measurements for evaluating the hemodynamic outcome of vascular surgery. Our results also suggest that it is extremely important to evaluate the outcome of such surgeries separately in patients with and without diabetes.

Torsion and bending analysis of internal fixation techniques for femoral neck fractures: the role of implant design and bone density

To evaluate pin/screw/plate fixation for management of femoral neck fractures, 39 proximal femora were tested in both torsion and flexion under physiological loading conditions. Three, four, or five implants of six commonly used multiple-fixation devices, and a sliding hip screw with and without an additional 6.5-mm cancellous screw were examined in paired femora. The intact and postfixation femora were initially subjected to a single applied moment, and the torsion and bending stiffness were determined from the load-deformation data. Postfixation femora were also subjected to cyclic loading in flexion at three load ranges, and fixation was judged successful if no failure occurred on or before 1,500 cycles of 667 to 2,000 N of a combined compressive force and moment. Anterior-posterior and lateral radiographs of each specimen were taken after fixation in order to evaluate Singh's index of bone density, fracture reduction, implant placement, and cross-sectional diameter of the femoral neck. Bone density was also evaluated by computed tomography (CT) and physical measurement of core samples obtained from the femoral head. The results indicate that there appears to be no justification for the use of more than three pin/screw implants for management of femoral neck fractures. Bone density was found to correlate with fracture stability and may be a useful predictor of fixation success.

Effects of chlorine dioxide on thyroid function in the African green monkey and the rat

In a previous study from this laboratory, chlorine dioxide (ClO2) treated drinking water depressed thyroxine (T4) levels in the African green monkey. The present study again demonstrated a decrease in T4 levels in the same species after 4 wk of oral exposure. However, after 8 wk of treatment T4 levels rebounded to above pretreatment levels, coinciding with an increase in thyroid radioiodide uptake. This T4 rebound phenomenon and increased iodide uptake may be due to a compensatory endocrinological mechanism. In rats, T4 levels dropped during the 8-wk ClO2 treatment period in a dose-dependent manner, and no rebound effect was observed. Iodide uptake values in the rat were not affected. It appears that ClO2 may have an effect on thyroid function in both species.

Effects of ClO2 on the absorption and distribution of dietary iodide in the rat

Aqueous chlorine dioxide (ClO2), an alternative disinfectant for drinking water, was found to decrease gastrointestinal (GI) bioavailability of dietary iodide. It has been previously reported that subchronic exposure to ClO2 decreases thyroxine (T4) levels in nonhuman primates. In this study in vitro experiments with animal feed, isolated rat stomachs, as well as in vivo studies with intact rats, showed that ClO2 in drinking water (at in situ concentrations as low as 2 ppm) oxidizes iodide to its reactive elemental (radical) state, binding it to organic substances present in the GI tract. A single instance of acute exposure to ClO2, however, did not decrease blood iodide levels, or thyroid glandular uptake of iodine.

Pressures on the foot in pointe shoes

This study was designed to determine the relative pressures on the first and second toes and first MTP joint when dancing en pointe, in addition to the variation of these pressures as a function of toe length, foot position, and padding. The first ray always bore the most pressure. Pressures on the second toe varied as a function of toe length and padding. Relevé in the everted position markedly increased first MTP joint pressures. Further research is required to determine whether properly designed padding and positioning of the foot en pointe could decrease the incidence of toe problems in the ballet dancer.

A comparison of convergent and bi-plane X-ray photogrammetry systems used to detect total joint loosening

X-ray photogrammetry is an accurate method of measuring structural displacements within the musculoskeletal system. Total joint implant alignment, migration and loosening are important clinical parameters which can be measured by X-ray photogrammetry. The purpose of this paper is to compare two different X-ray photogrammetry systems which are being used to study total joint parameters. The Seattle system uses a convergent geometry and the Cleveland system a bi-plane geometry. The accuracy of the two systems was compared by individually measuring the relative motion produced in an articulated plastic model. The model was designed to simulate the relative motion which can take place between the bone and a loose implant. The displacements of the model components were determined physically and photogrammetrically, and the error in the measurements was calculated for several conditions. Both systems measured relative component motion in the model with a root mean square error of 0.1 mm or less. In clinical use stainless steel markers are implanted in human subjects, and both systems measure distances with a r.m.s. error of 0.2 mm or less. The ease of usage, efficiency and flexibility of the two systems based on actual clinical experience reveals strengths and weaknesses in each which should be recognized when selecting a particular system.

Biomechanical study of the pitching elbow

Medial-tension injuries of the pitching elbow are well recognized. One contributing factor is the extreme valgus which has been noted to occur during the acceleration phase of throwing. It is hypothesized that breaking pitches generate higher medial loading because of the pronation and supination required to impart spin to the ball. The pitching motion is a complex action of all body segments to produce maximum linear and angular acceleration of the ball. The purpose of this study was to correlate elbow loading with pitching style. We measured the forearm segment for axial and tangential (varus-valgus plane) acceleration using accelerometers attached to the forearm and hand. Muscle activity was measured by EMG. Forearm rotation was assessed by stroboscopic photography. Despite different delivery styles when throwing breaking pitches, each pitcher demonstrated patterns of muscle activity and acceleration which were similar. Deceleration forces were lower than acceleration forces. Pronation and supination were documented and contribute to the direction of ball spin. Accelerometers can be used to evaluate pitching mechanics. We suggest that the main factors causing an elbow injury are the amount of throwing and the force with which the ball is thrown.