Intermittent reperfusion fails to prevent posttourniquet neurapraxia.

This study examined the effects of intermittent reperfusion on peripheral nerve function. Rabbits were randomized to undergo 4 hours of 350 mm Hg tourniquet compression to a hind limb either continuously, interrupted by a single 10-minute reperfusion interval after 2 hours, or interrupted by 10 minutes of reperfusion after each hour. A control group had the tourniquet applied for 4 hours but it was never inflated. The animals were examined clinically for neuromuscular dysfunction and the structure and function of the peripheral nerves were evaluated 1 week after tourniquet compression. Animals that underwent compression had a foot drop and decreased toe-spread reflex. There was greater intraneural edema and slower nerve conduction velocity in nerve segments that were directly compressed by the tourniquet but no apparent abnormalities in segments distal to the tourniquet. Intermittent reperfusion failed to diminish the clinical, structural, or functional consequence of the neurologic injury.

Effects of tourniquet compression on neuromuscular function.

Neuromuscular function in New Zealand White rabbits was evaluated after thigh tourniquet compression in the directly compressed quadriceps muscles and the distal tibialis anterior by measuring isometric contractile function after supramaximal stimulation of the motor nerve. Tourniquet compression resulted in markedly decreased force production beneath and distal to the tourniquet. Two days after compression, maximal quadriceps force production was decreased to 46% of control values with 125 mm Hg compression and 21% of control values after 350 mm Hg compression. Maximum tibialis anterior force production declined to 70% of control values after 125 mm Hg thigh compression and 24% of control values after 350 mm Hg thigh compression. Functional deficits were greater in the directly compressed quadriceps muscles, but the quadriceps and tibialis anterior had significantly increased impairment when the tourniquet inflation pressure was increased from 125 mm Hg to 350 mm Hg. Three weeks after compression, quadriceps function had returned to 94% of control value after 125 mm Hg compression and 83% after 350 mm Hg. Tibialis anterior function returned to 88% of control values after 125 mm Hg thigh compression and 83% after 350 mm Hg. Clinically, the use of lower inflation pressures may minimize the complications of tourniquet use and enhance postoperative recovery.

Growth hormone secretagogue increases muscle strength during remobilization after canine hindlimb immobilization.

Twenty-two beagles were divided into two equal groups, and the right hindlimb of each animal was immobilized at 105 degrees of knee flexion by external fixation. After 10 weeks of fixation, the device was removed, allowing free mobility for the following 5 weeks. Each day throughout the 15 weeks, one group received a growth hormone secretagogue (treatment) at a dose of 5 mg/kg, and the other received a lactose placebo (control). At weeks 0, 10, and 15, strength as indicated by maximum isometric extension torque was measured in the right hindlimb, biopsies of the vastus lateralis muscle were taken, and the dogs were weighed. Weekly blood samples were analyzed for insulin-like growth factor-1, blood urea nitrogen, and creatine phosphokinase. Between weeks 0 and 10, tetanic torque declined by about 60% (p < 0.001) in both groups, with no significant difference between the groups (p > 0.7). Between weeks 10 and 15, tetanic torque in the treated group increased by 0.81 Nm; this was significantly greater than the increase of 0.25 Nm in the placebo group (p < 0.05). The diameters of slow (type-1) and fast (type-2) fibers measured from the vastus lateralis muscle followed the same trend. At all time points, fiber diameter correlated strongly with torque; this argues against nonmuscular causes such as nerve injury for strength loss. The mean levels of insulin-like growth factor-1 increased 100% by week 4 in the treated group and remained elevated by about 60% throughout the experiment. Levels of insulin-like growth factor-1 in the placebo group decreased 30% within week 1 and remained depressed throughout the experiment. Our interpretation of these data suggests that the growth hormone secretagogue elevated levels of serum insulin-like growth factor-1, which in turn increased the size and strength of the quadriceps muscle during remobilization. These data may ultimately have therapeutic application to humans during rehabilitation after prolonged inactivity.

Intramuscular deoxygenation during exercise in patients who have chronic anterior compartment syndrome of the leg.

Currently, the definitive diagnosis of chronic compartment syndrome is based on invasive measurements of intracompartmental pressure. We measured the intramuscular pressure and the relative oxygenation in the anterior compartment of the leg in eighteen patients who were suspected of having chronic compartment syndrome as well as in ten control subjects before, during, and after exercise. Chronic compartment syndrome was considered to be present if the intramuscular pressure was at least fifteen millimeters of mercury (2.00 kilopascals) before exercise, at least thirty millimeters of mercury (4.00 kilopascals) one minute after exercise, or at least twenty millimeters of mercury (2.67 kilopascals) five minutes after exercise. Changes in relative oxygenation were measured with use of the non-invasive method of near-infrared spectroscopy. In all patients and subjects, there was rapid relative deoxygenation after the initiation of exercise, the level of oxygenation remained relatively stable during continued exercise, and there was reoxygenation to a level that exceeded the pre-exercise resting level after the cessation of exercise. During exercise, maximum relative deoxygenation in the patients who had chronic compartment syndrome (mean relative deoxygenation [and standard error], -290 +/- 39 millivolts) was significantly greater than that in the patients who did not have chronic compartment syndrome (-190 +/- 10 millivolts) and that in the control subjects (-179 +/- 14 millivolts) (p < 0.05 for both comparisons). In addition, the interval between the cessation of exercise and the recovery of the pre-exercise resting level of oxygenation was significantly longer for the patients who had chronic compartment syndrome (184 +/- 54 seconds) than for the patients who did not have chronic compartment syndrome (39 +/- 19 seconds) and the control subjects (33 +/- 10 seconds) (p < 0.05 for both comparisons).

Comparison of functional deficits in the rabbit tibialis anterior following tourniquet ischemia and tourniquet compression.

The purpose of this study was to measure the contractile properties of skeletal muscle after direct compression with a tourniquet and to compare these properties with those found after a similar period of tourniquet ischemia. A rabbit model of tourniquet compression of the tibialis anterior was developed and tested for this investigation. Fifty-seven animals then were divided into four protocol groups: (a) thigh tourniquet (ischemia) at 350 mm Hg (46.7 kPa), (b) leg tourniquet (compression) at 350 mm Hg (46.7 kPa), (c) leg tourniquet at 125 mm Hg (16.7 kPa), and (d) controls. A significant decrease in maximum tetanic tension was demonstrated in all three experimental groups. Direct compression at 350 mm Hg resulted in the greatest decline in maximum tetanic tension (22.6% of control), followed by compression at 125 mm Hg (30.5%) and ischemic injury (40.2%). In addition, direct compression at 350 mm Hg resulted in a significantly greater loss of force-generating capacity when compared with the ischemic group (p < 0.01). A similar pattern was noted for the rate of rise in maximum tetanic tension. Gross histologic examination of the tibialis anterior sections was consistent with the results of functional testing, with the more severe abnormalities noted in the compressed specimens. These results clearly demonstrate that tourniquet compression injury results in a more significant loss of functional strength and contractile speed than tourniquet ischemia. Further investigations on the safe limits of tourniquet use thus should be directed toward measuring the effect of the pneumatic tourniquet on the underlying soft tissues.

Pressure generation beneath a new thermoplastic cast.

A new cast immobilizer that is heat-shrunk to conform to an injured extremity was examined. The purpose of these studies was to compare pressures beneath the thermoplastic cast with those beneath fiberglass casts on a laboratory model and on the forearms of human volunteers. Pressures measured beneath fiberglass casts on metal cylinders averaged 36 mm Hg. Thermoplastic casts on the smaller cylinder that allowed 42% shrinkage produced a mean pressure of 25 mm Hg; those placed on the larger cylinder that allowed 17% shrinkage produced a mean pressure of 39 mm Hg. Pressures measured on the forearms of healthy volunteers averaged 22 mm Hg beneath fiberglass casts and 31 mm Hg beneath the thermoplastic casts. These pressures were considerably less than pressures that have been shown to occlude the microcirculation of the skin. Acute compartment syndromes result from swelling within a limited space and remain a serious concern clinically when swelling is anticipated under any type of constraining cast. The results of these studies indicate that the new cast should not produce a greater risk of circulatory compromise to the limb than previously used fiberglass materials.

Effects of an antioxidant in a rabbit model of tourniquet-induced skeletal muscle ischemia-reperfusion injury.

The purpose of this study was to evaluate the effects of U74006F, a new antioxidant, on isometric contractile function after tourniquet-induced ischemia-reperfusion injury. Twenty-six NZW rabbits underwent either 2 or 4 hr of thigh tourniquet compression. Animals were randomized to receive U74006F or an equal volume of its citrate vehicle. When tibialis anterior function was tested 2 days later, there was no difference between treatment groups in peak tension, rate of force production, contraction time, half relaxation time, or resistance to fatigue. The use of U74006F failed to improve functional outcome in this model of ischemia-reperfusion injury.

Effects of a functional knee brace on leg muscle function.

The effects of a functional knee brace on local intramuscular pressures and on calculated blood perfusion pressure in the tibialis anterior muscle of the legs of six volunteers were determined. Torque generation during dorsiflexion of the ankle joint was measured with a Kinetic Computerized ergometer, and the time to elicit muscle fatigue during exercise was recorded. For each experimental subject studies compared right with left unbraced legs, then right braced with left unbraced legs, and finally right unbraced with left braced legs. In the braced leg, intramuscular pressure at rest and muscle relaxation pressure during exercise were significantly higher, and the time to elicit muscle fatigue was 35% shorter. The calculated local blood perfusion pressure was lower because of the increased muscle relaxation pressure. We conclude that the increased muscle relaxation pressure during exercise caused by a knee brace is a possible explanation for the premature development of muscle fatigue in the braced leg.

Effect of ankle position and a plaster cast on intramuscular pressure in the human leg.

Intramuscular pressure was measured with transducer-tipped catheters that had been inserted into the anterior and deep posterior compartments of the leg in seven healthy adults. Intramuscular pressure increased three to sevenfold (depending on the position of the ankle) in both compartments after the application of a plaster cast from the proximal part of the thigh to the malleoli. While the cast was in place, the baseline intramuscular pressure was elevated by the inflation of a tourniquet, which was located on the proximal part of the thigh, to a pressure of sixty millimeters of mercury (8.00 kilopascals). The intramuscular pressure in both the anterior and the deep posterior compartments was found to be lowest when the ankle joint was between the neutral and the resting positions (between 0 and 37 degrees of flexion). After the cast was bivalved and the opening on each side was spread approximately one-half centimeter, there was a significant decrease in intramuscular pressure of 47 per cent in the anterior compartment and of 33 per cent in the deep posterior compartment (p < 0.05 for both).

Muscle functional deficits after tourniquet ischemia.

The contractile properties of the rabbit tibialis anterior muscle were studied 48 hours after an ischemic episode induced by pneumatic tourniquet compression of the thigh. Forty animals were divided into five groups, each of which had continuous ischemia of either 1, 2, or 4 hours, or a total of 2 or 4 hours of ischemia interrupted by 10 minutes of reperfusion at 1-hour intervals. Contralateral limbs served as controls. Muscle contractile properties were tested by stimulation of the peroneal nerve distal to the site of tourniquet compression. Peak tetanic tension in the 1-hour group did not differ significantly from controls. In the 2- and 4-hour groups, peak tetanic tensions were 31% and 2% of controls, respectively, and twitch tensions were 25% and 1% of controls, respectively. Hourly reperfusion intervals had no significant effect on maximum tetanic or twitch tension compared with continuous ischemia for either 2 or 4 hours. Clinically significant muscle dysfunction may be induced by 2 or more hours of pneumatic tourniquet application. Hourly reperfusion intervals may not improve skeletal muscle function distal to the tourniquet. However, reperfusion intervals could still affect muscle that is compressed beneath the cuff. Tourniquet-induced contractile deficits may interfere with postoperative functional recovery.

Intramuscular pressure varies with depth. The tibialis anterior muscle studied in 12 volunteers.

Pressures in the tibialis anterior muscle were recorded at rest and during exercise with transducer-tipped catheters in 12 volunteers while they were supine or standing. The recordings were repeated with venous stasis created by an inflated tourniquet cuff on the thigh. Catheters were placed at 3 different sites in the muscle: catheter I adjacent to the deep surface of the fascia over the anterior compartment; catheter II between the fascia and the central tendon; and catheter III deep in the muscle close to the interosseous membrane. In both the supine and standing positions the intramuscular pressure at rest and the muscle relaxation pressure during exercise, obtained by catheter II, were greater than the corresponding pressures measured by the superficially located catheter I in the normal as well as in the volume loaded limb. The same conditions for pressure measurement consistently revealed lower pressures recorded by catheter III compared to II, but the difference was not significant. Our results indicate that intramuscular pressure increases centripetally, as the centrally lying tendon is approached. We conclude that pressure measurements for diagnosis of acute and chronic compartment syndromes and in ergonomic studies should be based on recordings from a standard location of the catheter within the muscle and a standard posture of the subject.

The use of lower tourniquet inflation pressures in extremity surgery facilitated by curved and wide tourniquets and an integrated cuff inflation system.

Use of the lowest possible cuff inflation pressure should minimize the pathogenic effects of compression beneath the pneumatic tourniquet. Curved tourniquets (designed to fit conically shaped limbs) and wider tourniquets were associated with significantly lower arterial occlusion pressure (AOP) than standard, straight tourniquets on the arms and legs of 26 normal volunteers. These tourniquets were used with an integrated tourniquet inflation system in 29 upper-extremity and 31 lower-extremity surgeries. Mean tourniquet inflation pressures of 183.7 mm Hg and 208 mm Hg were used during various surgical procedures of the arm and leg, respectively. Incomplete hemostasis was associated with elevated systolic blood pressure in several cases, but acceptable surgical hemostasis was achieved by incremental increase of the cuff inflation pressure. Curved cuffs, wide cuffs, and an integrated cuff inflation system should facilitate the use of lower tourniquet inflation pressures in extremity surgery.

The effects of non-weight-bearing and limited motion on the tensile properties of the meniscus.

Recent clinical studies have suggested that many of the complications of prolonged immobilization after knee surgery can be prevented by permitting early motion while minimizing loading of healing tissues. The purpose of this study was to determine the effects of such a regimen on the tensile properties of the meniscus. The right knee of 10 skeletally mature sheep received a sham operation after which the hindlimb was placed in a harness that prevented weight bearing while permitting limited knee motion. The left knee served as the nonoperated control. Twelve weeks after surgery, paired specimens from the medial meniscus of each limb were tested in tension with the load parallel to the circumferentially oriented collagen fibers. No significant differences in the modulus, tensile strength, and ultimate strain between treated and control medial menisci were identified. Our data thus suggest that preventing weight bearing while permitting even limited motion of the knee will prevent any detrimental effect on the tensile properties of the meniscus in the circumferential direction.

Effects of reperfusion intervals on skeletal muscle injury beneath and distal to a pneumatic tourniquet.

To date there have been no experimental studies specifically directed at effects of reperfusion intervals on skeletal muscle injury beneath the tourniquet. 99mTechnetium pyrophosphate (Tc 99) incorporation and correlative histology were used to assess injury 2 days after tourniquet application in muscles beneath (thigh) and distal (leg) to the cuff. Tourniquets were applied to rabbit hindlimbs for a total of either 2 or 4 hours. In the 4-hour series, tourniquet compression (either 125 mm Hg or 350 mm Hg cuff inflation pressure) was either continuous or interrupted by 10-minute reperfusion intervals after 2 hours or after every hour of cuff inflation. In the 2-hour series, tourniquet compression (350 mm Hg) was either continuous or interrupted by 10-minute reperfusion intervals after 2 hours or after every hour of cuff inflation. In the 2-hour series, tourniquet compression (350 mm Hg) was either continuous or interrupted by a 10-minute reperfusion interval after 1 hour. Pyrophosphate incorporation (Tc 99 uptake) was significantly greater in the thigh region than in the leg region in all of the 4-hour tourniquet groups. Tc 99 uptake was significantly reduced by reperfusion after each hour of cuff inflation. With 350 mm Hg tourniquet pressure, a reperfusion interval after 2 hours of cuff inflation tended to exacerbate tourniquet compression injury. Reperfusion intervals did not significantly affect Tc 99 uptake in the leg region of these groups. With a 2-hour tourniquet time, Tc 99 uptake in the thigh was significantly decreased by reperfusion after 1 hour of cuff inflation. Previous clinical recommendations, based on serum creatine phosphokinase abnormalities after experimental tourniquet ischemia, probably reflected tourniquet compression injury. Hourly reperfusion limits skeletal muscle injury during extended periods of tourniquet use.

Functional knee braces increase intramuscular pressures in the anterior compartment of the leg.

The effect of three different functional knee braces on intramuscular pressures in the anterior compartment of the leg was investigated in 8 healthy subjects. Pressures were recorded with the microcapillary infusion technique while the subjects were either supine, sitting, or standing. Pressures at rest in the anterior tibial muscle increased significantly following application of each of the three knee braces regardless of posture. Similarly, muscle relaxation pressure during exercise also increased significantly on brace application. A pressure of 40 mm Hg was exceeded in 9 of 18 intramuscular pressure measurements with the subject standing. The tested functional knee braces increased muscle pressures at rest and muscle relaxation pressure during exercise to levels that, according to other studies, might decrease muscle blood flow significantly. External compression from a knee brace on leg muscles might, therefore, induce premature muscle fatigue because of local insufficient perfusion of the working muscle.

Decreased muscle speed, strength and fatigability following two hours of tourniquet-induced ischaemia.

Contractile and morphological properties of the rabbit tibialis anterior muscle were measured 48 hours following a two-hour ischaemic episode. Ischaemia was induced using a specially-designed pneumatic tourniquet placed on the rabbit thigh. Maximum tetanic tension of muscle subjected to ischaemia (381 +/- 77 g) was only about 30% of the tension generated by control muscles (1,212 +/- 67 g). The rate of rise of tetanic tension of muscles subjected to ischaemia (15.9 +/- 3 g/ms) was only 33% of control values (44.5 +/- 5.9 g/ms). Muscle fatigue index increased significantly from 0.22 +/- 0.7 in control muscles to 0.55 +/- 0.09 in ischaemic muscles suggesting that muscles subjected to ischaemia had a greater endurance capacity than control muscles. Morphologically, focal necrotic regions and inflammatory cells were observed in ischaemic muscle fibers. Taken together, these data are consistent with selective damage to the fast glycolytic muscle fibers within the ischaemic tibialis anterior muscles. Thus, ischaemia results in overall decreased muscle speed, strength and fatigability.

Viscoelastic shear properties of the equine medial meniscus.

Recent studies have shown that the meniscus is highly anisotropic in tension and that its compressive creep behavior can be modeled using biphasic theory. In this study, an alternative approach is used, where viscoelastic shear properties of the meniscal fibrocartilage are measured to determine the anisotropy and inhomogeneity of this tissue with respect to specimen location and fiber orientation. Medial menisci were obtained from eight skeletally-mature horses. Nine test specimens were taken from the circumferential midsubstance of each meniscus, at three circumferential and three axial positions. The magnitude of the complex shear modulus and the phase angle were determined for each specimen from 100-800 Hz, in 100 Hz increments. Data were gathered shearing parallel and perpendicular to the circumferentially-oriented fibers. The magnitude of the shear modulus and the phase angle were both found to be frequency dependent, anisotropic, and inhomogeneous. The magnitude of the shear modulus increased with frequency, and was greatest in specimens from the posterior superior region, shearing parallel to the fibers. The phase angle decreased slightly with frequency and was lowest in specimens from the midsubstance of the anterior region, shearing perpendicular to the fibers. Our data demonstrated that collagen fibers substantially stiffen the meniscus in the direction of its fibers and that the solid matrix of the meniscus, like articular cartilage, behaves largely as an elastic material.

Muscle injury induced beneath and distal to a pneumatic tourniquet: a quantitative animal study of effects of tourniquet pressure and duration.

Previous recommendations regarding the "safe" period of tourniquet hemostasis were based largely on studies of ischemia distal to the tourniquet. This study quantitatively analyzed skeletal muscle injury induced beneath and distal to a pneumatic tourniquet applied to the hindlimbs of rabbits for 1, 2, or 4 hours with a cuff inflation pressure of 125, 200, or 350 mm Hg. Technetium Tc 99m pyrophosphate incorporation after systemic injection (Tc 99 uptake) and correlative histology were used to evaluate tissue damage 2 days after tourniquet application. Compared with the contralateral control limbs, compression and ischemia induced statistically significant increases in Tc 99 uptake in the thigh and leg regions of all groups. Pyrophosphate incorporation was significantly greater in the thigh region than in the leg region after 2 hours of compression in the 200 and 350 mm Hg pressure groups and following 4 hours of compression in all pressure groups. Focal and regional fiber necrosis and degeneration were observed in thigh muscles after 2 hours of tourniquet compression. Two hours of continuous tourniquet application at clinically relevant cuff inflation pressures induced significant skeletal muscle necrosis beneath the tourniquet. Use of the lowest possible inflation pressure for a limited duration should minimize the degree of tissue injury caused by tourniquet application.

Effects of an in-substance conduit with injection of a blood clot on tears in the avascular region of the meniscus.

The purpose of this study was to test the hypothesis that healing of a stable tear in the avascular region of the meniscus occurs when a horizontal conduit, extending from the periphery to the defect, is filled with an exogenous fibrin clot. In 6 sheep a full-thickness laceration was made in the lateral meniscus, and autologous blood clot was then injected into the conduit. Three animals in the control group received identical meniscal tears but no additional treatment. Casting and a harness prevented weight-bearing and maintained the knee in a flexed position. Twelve weeks after the operation histologic examination revealed only partial healing in one animal. In both the experimental and control groups increased numbers of dividing chondrocytes on either side of the tear were seen. We conclude that an in-substance conduit in combination with the injection of a blood clot is not sufficient to facilitate complete healing of a tear within the avascular region of the lateral meniscus of the sheep when the knee is immobilized only with a harness.

An animal model for the study of neuromuscular injury induced beneath and distal to a pneumatic tourniquet.

A well-controlled animal model is presented for the study of neuromuscular injury induced by a pneumatic tourniquet. This model comprises a curved tourniquet surrounded by a stiff exterior shell, both of which were specifically designed to fit the conical and oblong shape of the rabbit hindlimb. Computed tomographic imaging was used to assess transverse tissue displacement induced by tourniquet compression. The curved tourniquet/shell configuration occluded the distal arterial blood flow to the extremity at a significantly lower cuff inflation pressure than a straight tourniquet of equal width. The magnitude and distribution of tissue pressures in the subcutaneous and deep tissues beneath the tourniquet were similar to those recorded in previous human cadaver studies of tourniquet compression. This animal model will facilitate the quantitation and analysis of tissue injury induced beneath and distal to a pneumatic tourniquet. Such data can help define the critical pressure and time limits for the safe use of pneumatic tourniquets in extremity surgery.