Anterior and posterior musculotendinous anatomy of the supraspinatus.

The objective of this study was to quantitatively describe the supraspinatus musculotendinous architecture. After supraspinatus muscles were harvested from 25 embalmed shoulders, each muscle was divided into an anterior and posterior muscle belly on the basis of muscle fiber insertion. Pennation angles and musculotendinous dimensions were measured, and the physiologic cross-sectional area was calculated for each muscle belly. The physiologic cross-sectional areas of the anterior and posterior bellies were calculated to be 140 +/- 43 mm2 and 62 +/- 25 mm2, respectively, whereas their tendon cross-sectional areas were 26.4 +/- 11.3 mm2 and 31.2 +/- 10.1 mm2, respectively. The average anterior-to-posterior ratios for the muscle physiologic cross-sectional area and the tendon cross-sectional area were 2.45 +/- 0.82 and 0.87 +/- 0.30, respectively. Thus, a larger anterior muscle pulls through a smaller tendon area. These data suggest that physiologically, anterior tendon stress is significantly greater than posterior tendon stress and that rotator cuff tendon repairs should incorporate the anterior tendon whenever possible, inasmuch as it functions as the primary contractile unit.

The incidence of ganglion cysts and other variations in anatomy along the course of the suprascapular nerve.

When the diagnosis of suprascapular nerve entrapment syndrome is being considered, variations in anatomy are possible etiologic factors. Seventy-nine shoulders from 41 cadavers were examined for anatomic variations and for ganglion cyst formation in the suprascapular notch, superior transverse scapular ligament, and inferior transverse scapular ligament. The morphologic evaluation of the suprascapular notch revealed a "U" shape in 77% and a "V" shape in 23%, with 89% of cadavers having the same notch shape bilaterally. In 23% of shoulders a variation of the superior transverse scapular ligament was demonstrated such as partial and complete ossification and multiple bands including the first report of a trifid superior transverse scapular ligament. An inferior transverse scapular ligament was observed in only 14% of shoulders. One ganglion cyst was identified, for an incidence of 1%. The mass was located in the supraspinatus fossa adjacent to the superior transverse scapular ligament and appeared to compress and alter the course of the suprascapular nerve. When operative treatment is elected for suprascapular nerve entrapment syndrome and an open surgical approach is undertaken, the location and source of disease and morphologic and anatomic variants must be recognized to ensure adequate access and complete decompression of the suprascapular nerve. The classical description of the superior transverse scapular ligament as a completely nonossified single band should be expected, on average, in approximately three fourths of the cases. Partial or complete ossification and anomalous bands of the superior transverse scapular ligament or a ganglion cyst along the course of the suprascapular nerve may be encountered. Although a superior transverse scapular ligament should be anticipated in all shoulders, an inferior transverse scapular ligament will be a much less frequent finding. The role and significance of suprascapular notch morphologic characteristics warrant further investigation.

Patellar stabilization: a quantitative evaluation of the vastus medialis obliquus muscle.

Twenty-one cadaveric knees were dissected to analyze the functional anatomy of the vastus medialis complex (VMC), which is composed of the vastus medialis obliquus (VMO) and the vastus medialis longus (VML) muscles. The physiologic cross-sectional area of the VMO for 20 of the knees was 29% of the total physiologic cross-sectional area for the VMC. These values differed in one knee with a dysplastic VMO. The quantitative description of the VMO provided by this study will facilitate future efforts to accurately model the physiologic function of the VMO in cadaveric investigations on patellofemoral mechanics. The effect of simulated pathologies and surgical reconstructions then may be determined with more certainty to improve patient management.

A common epineural sheath for the nerves in the popliteal fossa and its possible implications for sciatic nerve block.

Sciatic nerve block in the popliteal fossa is associated with a highly variable success rate. Frequently, anesthesia is profound in the distribution of both the tibial (TN) and common peroneal nerves (CPN), although the response to nerve stimulation or paresthesia is obtained in the distribution of one division of the nerve. However, anesthesia in the distribution of only one division of the nerve is also a common occurrence under apparently identical clinical circumstances. Looking for a possible role of a common epineural sheath in these phenomena, we injected dye into the epineural sheath of the tibial nerve in 10 cadaver legs and observed its spread within the sheath. Injections of 15 mL and 30 mL of the dye resulted in a proximal spread of 147 +/- 34 mm and 172 +/- 50 mm, respectively, from the injection point 10 cm below the popliteal fossa crease. In a majority of the legs, the dye reached the division of the sciatic nerve in the popliteal fossa, bathing both the TN and CPN. Gross inspection and histologic examination of the sciatic nerve specimens revealed a common epineural sheath enveloping the TN and CPN. The presence of the common epineural sheath and its characteristics may have important clinical implications for sciatic nerve blockade in the popliteal fossa.

Anatomic considerations for sciatic nerve block in the popliteal fossa through the lateral approach.

BACKGROUND AND OBJECTIVES: The disadvantage of the classic posterior approach to block of the sciatic nerve at the knee level (popliteal nerve block [PNB]) is the need to position a patient in the prone position for performance of the block. In this study on cadavers, a lateral approach to the popliteal nerve in the supine position was investigated, and some anatomic considerations of relevance to popliteal nerve block were addressed. METHODS: In 19 cadaver right legs, the lateral approach to PNB was simulated with a needle, introduced in the groove between the biceps femoris and vastus lateralis muscles 7 cm above the knee, at either 30 degrees or 60 degrees relative to the horizontal plane, and 1 mL of dye solution was injected through the needle. After dissection of the popliteal fossa, the position of the solidified bolus of dye in relation to the popliteal nerve was determined. Additionally, the dye was injected into the popliteal nerve sheath, and the spread of the dye and continuity of the sheaths were determined. RESULTS: In 10 legs, the lateral approach was attempted at a 30 degrees angle and in 9 legs at a 60 degrees angle. The solidified injectates at 30 degrees were closely distributed anterolaterally to the nerve, while injectates at 60 degrees tended to be further from the nerve and scattered along its posterolateral aspect (P = .02). The dye injected into the nerve sheaths traveled 5 to 10 cm within the sheath, surrounding both main divisions of the popliteal nerve, the tibial and the common peroneal nerve. CONCLUSION: A lateral approach to the popliteal nerve with insertion of the needle at a 30 degrees angle relative to the horizontal plane results in predictable approximation of the needle tip to the popliteal nerve. The results also suggest the existence of a continuous neural sheath encompassing the popliteal nerve and its main branches. This may have clinical implications similar to those in perivascular neuronal block.

An anatomic study of the subscapular nerves: A guide for electromyographic analysis of the subscapularis muscle.

Fifty shoulders from 36 human cadavers were examined to identify the nerves innervating the subscapularis muscle and their point of entry into the muscle. Most of the specimens (82%) revealed three independent nerves to the subscapularis, 16% of the specimens demonstrated four nerves, and 2% of the shoulders demonstrated two nerves to the subscapularis. Variability was noted at the level of origin (division or cord) of each primary nerve branch to the muscle. The point of entry of each nerve into the subscapularis was measured from three clinical anatomic landmarks. The entry points were found to closely follow a line running parallel to the vertebral border of the scapula and inferior from the medial surface of the base of the coracoid. Previous electromyogram studies of the subscapularis have used one electrode to study its dynamic function. The anatomic data in this and other series suggest that multiple electrodes may be required for a complete electromyogram study of the subscapularis. The findings of this study facilitate the placement of two intramuscular fine wire electrodes for electromyogrophic analysis, which permits the investigation of the subscapularis muscle as two functional units.

Tibialis posterior muscle: the fifth compartment?

The posterior aspect of 51 embalmed cadaver legs in 50 cadavers was dissected to establish the prevalence of a separate compartment for the tibialis posterior muscle (TP). All dissections revealed the presence of a superficial and a deep posterior compartment. No distinct fascial septum separated the TP from the flexor digitorum longus (FDL) and flexor hallucis longus (FHL). We conclude that the TP does not commonly rest within its own osseofascial compartment and thus does not require isolated decompression for acute compartment syndrome of the leg. An incidental observation, frequently overlooked in the anatomy literature, was a supplemental tendon of origin of the FDL. In addition to the classically described tibial origin, several cadavers exhibited a proximal fibular tendon of origin for the FDL. One cadaver demonstrated the FDL to have an extensive fibular origin that completely covered the TP, forming a myotendinous fifth compartment. We feel that the variable fibular origin can explain the chronic exertional compartment syndrome of the TP described previously by Davey et al. and serves as a basis for a minor alteration in our fasciotomy technique.

An anatomical study of the suprascapular nerve.

The course of the suprascapular nerve and its distance from fixed scapular landmarks were measured in 90 cadaveric shoulders. In an additional 15 cadavers, three pins were passed at various angles in a general anterior-posterior direction through the middle of the glenoid neck just inferior and lateral to the base of the coracoid process. The distance between the exit site on the posterior glenoid neck and the suprascapular nerve at the base of the scapular spine was recorded for each pin. Inferiorly directed pins were the furthest from the suprascapular nerve and averaged 16 mm. On the basis of these data, a relative safe zone is described in the posterior glenoid neck. Knowledge of the anatomic course of the suprascapular nerve may aid the physician in the diagnosis and treatment of suprascapular neuropathies. Appreciation of the safe zone may help the shoulder surgeon avoid iatrogenic injury to the suprascapular nerve during arthroscopic Bankart procedures and other open surgical procedures requiring dissection of the posterior glenoid neck.

An anatomic study of the musculocutaneous nerve and its relationship to the coracoid process.

The anatomy of the musculocutaneous nerve was investigated to determine its relationship to the coracoid process. The distance from the coracoid to the point of entrance of the main nerve trunk into the coracobrachialis muscle ranged from 31 to 82 mm, with a mean of 56 mm. Small nerve twigs to the coracobrachialis (proximal to the main nerve trunk) entered the muscle as close as 17 mm below the coracoid, with an average of 31 mm. The frequently cited range of 5-8 cm below the coracoid for the level of penetration cannot be relied on to describe a safe zone because 29% of the nerves entered the muscle proximal to 5 cm below the coracoid (74% if the proximal twigs are counted). If coracoid mobilization is necessary, the musculocutaneous nerve and its twigs should be identified and protected, keeping in mind the variations in anatomy and the level of penetration.

Donnan potentials from striated muscle liquid crystals. Lattice spacing dependence.

Electrochemical potentials were measured as a function of myofilament packing density in crayfish striated muscle. The A-band striations are supramolecular smectic B1 lattice assemblies of myosin filaments and the I-band striations are nematic liquid crystals of actin filaments. Both A- and I-bands generate potentials derived from the fixed charge that is associated with structural proteins. In the reported experiments, filament packing density was varied by osmotically reducing lattice volume. The electrochemical potentials were measured from the A- and I-bands in the relaxed condition over a range of lattice volumes. From the measurements of relative cross-sectional area, unit-cell volume (obtained by low-angle x-ray diffraction) and previously determined effective linear charge densities (Aldoroty, R.A., N.B. Garty, and E.W. April, 1985, Biophys. J., 47:89-96), Donnan potentials can be predicted for any amount of compression. In the relaxed condition, the predicted Donnan potentials correspond to the measured electrochemical potentials. In the rigor condition, however, a net increase in negative charge associated with the myosin filament is observed. The predictability of the data demonstrates the applicability of Donnan equilibrium theory to the measurement of electrochemical potentials from liquid-crystalline systems. Moreover, the relationship between filament spacing and the Donnan potential is consistent with the concept that surface charge provides the necessary electrostatic force to stabilize the myofilament lattice.

Active force as a function of filament spacing in crayfish skinned muscle fibers.

Filament spacing is shown to have a pronounced effect on active force in skinned striated muscle fibers of crayfish. At constant filament overlap and constant ionic strength, the separation between the myofilaments (measured by low-angle X-ray diffraction) was adjusted by application of osmotic pressure. Force was induced by a calcium-containing activating solution. In the absence of compression, calcium-activated force in skinned fibers was approximately 80% of that in normal intact fibers. In fibers compressed somewhat beyond the dimension of intact fibers, force was maximal. With further compression, force was reduced and then abolished. The filament spacing-force relation reported here suggests that, at any instant, the distance between the myosin filaments and actin filaments affects either the axial force per cross bridge or, more likely, the number of cross bridges in the force-generating state.

Donnan potentials from striated muscle liquid crystals. Sarcomere length dependence.

Donnan potentials from A-bands and I-bands were measured as a function of sarcomere length in skinned long-tonic muscle fibers of the crayfish. These measurements were made using standard electrophysiological technique. Simultaneously, the relative cross-sectional area of the fibers was determined. Lattice plane spacings and hence unit-cell volumes were determined by low-angle x-ray diffraction. At a sarcomere length at which the myosin filaments and actin filaments nominally do not overlap, measurements of potential, relative cross-sectional area, and unit-cell volume were used in conjunction with Donnan equilibrium theory to calculate the effective linear charge densities along the myosin filament (6.6 X 10(4) e-/mu) and actin filament (6.8 X 10(3) e-/mu). Using these linear charge densities, unit-cell volumes and Donnan equilibrium theory, an algorithm was developed to predict A-band and I-band potentials at any sarcomere length. Over the range of sarcomere lengths investigated, the predicted values coincide with the experimental data. The ability of the model to predict the data demonstrates the applicability of Donnan equilibrium theory to measurements of electrochemical potential from liquid-crystalline systems.

Donnan potentials from striated muscle liquid crystals. A-band and I-band measurements.

A-band and I-band potentials are measured selectively in crayfish skinned long-tonic muscle fibers. The microelectrode tip diameters used in this study are shown to be sufficiently small to permit the discrete placement of an electrode into either an A-band or I-band. Random and directed impalements into mechanically skinned fibers with microelectrodes yields reproducible trimodal distributions of potentials where the modalities represent the A-band (-1.80 mV), the I-band (-0.76 mV), and the Z-line vicinity (-3.63 mV). In conjunction with Donnan equilibrium theory, fixed charge concentrations are calculated from the measured potentials for the A-band (25.9 mmol e-/l), I-band (10.9 mmol e-/l), and Z-line vicinity (52.3 mmol e-/l). When skinned fibers are treated with Triton X-100, the mean potentials (and charge concentrations) decrease: A-band to -1.71 mV (24.6 mmol e-/l), I-band to -0.71 mV (10.2 mmol e-/l), and the Z-line vicinity to -3.40 mV (49.0 mmol e/l). In the A-band this represents a loss of 1.3 mmol e-/l while in the I-band 0.7 mmol e-/l are lost; both decreases are attributed to the removal of internal membranous structures. In the rigor condition the A-band increases to -2.18 mV (33.1 mmol e-/l) and the I-band increases to -0.88 mV (13.3 mmol e-/l). Relative to the relaxed condition, this represents an increase of 8.5 mmol e-/l and 3.1 mmol e-/l in the A-band and I-band, respectively. The evidence shows that it is practical to measure A-band and I-band potentials selectively. Further, it is demonstrated that similar measurements can be obtained from agar, another polyelectrolyte gel system (see Appendix).

Degenerative changes at the neuromuscular junctions of red, white and intermediate muscle fibers. Part 1. Response to short stump nerve section.

Degeneration at the neuromuscular function following cutting the phrenic nerve at the 9th intercostal space differs in red, white and intermediate skeletal muscle fibers. The ultrastructure of the nerve terminal and the muscle fiber between 12 hours and 21 days following denervation suggests that lack of neurotrophic influences results in responses specific for each fiber type. Degeneration of axon ends is rapid and by 2 days axon terminals are missing from the end-plate areas of all 3 fiber types. Schwann cells "engulf" degenerating axon terminals and eventually replace them in the primary clefts. Schwann cells display specific morphological changes directly related to axonal degeneration. In all instances axon terminal degeneration precedes muscle fiber degeneration. Synaptic cleft changes are similar for all types of muscle fibers. Primary cleft structure appears to be dependent upon neurotrophic influence, whereas secondary cleft structure is relatively unaffected by denervation. Initial changes in subsynpatic regions of muscle fibers include focal loss of sarcomere alignment and skewing of the Z lines. By 21 days myofibrillar disorganization appears most severe in white fibers and least in red muscle fibers. The rate and degree of degeneration of the axon terminal and subjacent muscle fiber are different for each of the 3 muscle fiber types.

Degenerative changes at the neuromuscular junctions of red, white, and intermediate muscle fibers. Part 2. Responses to long stump nerve section and colchicine treatment.

The ultrastructure at the neuromuscular junction (NMJ) of red, white, and intermediate skeletal muscle fibers undergoes specific changes following either unilateral severance of the phrenic nerve or unilateral topical treatment of the phrenic nerve with colchicine. Both procedures were performed in the cervical region and produce similar rates of muscle fiber degeneration. The severity of degeneration appears to be related to muscle fiber type with white fibers being most severely affected and red fibers least affected. Degeneration rates of the axon terminal also correlate with fiber type in the orderwhite, intermediate, red. However, the rates of degeneration of the specific axon terminals are more rapid with surgical severance than with colchicine treatment. Statistical analysis of morphometric data indicates that hemidiaphragms denervated surgically exhibited significant axon terminal degeneration before significant muscle degeneration. Conversely, diaphragmatic muscle fibers of colchicine-treated phrenic nerves exhibit significant degeneration before loss of the axon terminal. Despite reversal of the temporal sequences for loss of axonal and muscular components between the two preparations, degenerative characteristics of muscle fiber structure are similar. This suggests that the presence of fiber-specific neurotrophic substances transmitted from the neuronal cell body to the axon terminal and released at the NMJ may be an important factor in the maintenance of normal muscle fiber morphology.

The myofilament lattice: studies on isolated fibers. IV. Lattice equilibria in striated muscle.

Accounts of similarities between the thick filament lattice of striated muscle and smectic liquid-crystalline structures have focused upon an equilibrium between electrostatic (repulsive) and van der Waal's (attractive) forces. In living, intact muscle the fiber volume constitutes an additional important parameter which influences the amount of interaxial separation between the filaments. This is demonstrable by comparison of the lattice behavior of living fibers with that of fibers from which the sarcolemma has either been removed or made leaky by glycerination. These comparisons were made mainly by low-angle X-ray diffraction under conditions of changes in sarcomere length, ionic strength or osmolarity, and pH. Single fibers with the sarcolemma removed and glycerinated muscle have lattices which behave in accord with equilibrium liquid-crystalline systems in which the thick filament spacing is determined by the balance between electrostatic and van der Waal's forces. Conversely, osmotic and shortening studies demonstrate that the living, intact muscle has a lattice which behaves in accord with the so-called non-equilibrium (volume-constrained) liquid-crystalline condition in which the interaxial separation between the thick filaments is solely due to the amount of volume available as determined by the Donnan steady-state across the sarcolemma.

The myofilament lattice: studies on isolated fibers. 3. The effect of myofilament spacing upon tension.

The effect of ionic strength on the generation of tension and upon the interfilament spacing in living intact and skinned single striated muscle fibers from the walking leg of crayfish (Orconectes) were determined by isometric contraction studies correlated with low-angle X-ray diffraction. Sarcomere lengths were determined by light diffraction. Tensions were induced in intact fibers by caffeine in the bathing medium and by ionophoretic microinjection of calcium. Tensions were induced in skinned fibers by a buffered calcium-EGTA solution. The interfilament spacing of intact and skinned fibers over the range of ionic strengths investigated were determined by X-ray diffraction and correlated with the physiological data. It is demonstrated that the ionic strength affects the tension-generating capacity of the muscle as it affects the chemo-mechanical transform of excitation-contraction coupling. It is further demonstrated that interfilament spacing changes encountered during shortening and with variation in the osmotic strength have no effect upon the tension-generating capacity of muscle.