The first description of an isolated subscapularis tendon tear is attributed to Gerber and Krushell.1 The authors noted that clinical diagnosis of subsapularis tears remains a challenge and they described the so-called lift-off sign as a reliable clinical sign for the diagnosis of subscapularis insufficiency. In a subsequent follow-up of his experience Gerber reported on the midterm results after repair of the subscapularis tendon. He observed that repairs of chronic subscapularis tears had a much poorer outcome than repairs performed in an acute setting. 2
The anterosuperior cuff tear configuration, which represents a tear of the subscapularis in combination with the supraspinatus and sometimes the infraspinatus, was first recognized as a discrete entity by Warner et al. .3 The authors observed, in the same way as Gerber, that tears involving the subscapularis often had a delayed diagnosis which resulted in late presentation of the patients for treatment.
Indication for reconstruction of chronic subscapularis tears by tendon tranfer are not yet fully established. All considerations must be placed in the context of the patients disability and their expectations for pain relief and functional recovery. Many factors like location of the tear, quality of the tendon tissue to repair, associated degenerative changes of the glenohumeral joint, number and nature of previous surgeries, age and compliance of the patient should be considered prior surgery. As this tear configuration occurs usually in younger and active patients, treatment in the chronic situation after delayed diagnosis is challenging, because recovery of function and strength is essential in this high demanding group of patients.
Structural integrity of the rotator cuff is a conditio sine qua non for normal shoulder function.4-6 Although clinical results after repair of massive rotator cuff tears are frequently good 7,8, several studies have shown that structural healing does not reliably occur after technically successful repair of massive tears.4 ,5,9
During many years the principles in diagnosis and treatment of rotator cuff tears had focused on the tendinous defect and its reattachment to the bone only, without considering the effect of the tendon tear on the corresponding muscle.
The correlation between rotator cuff tear and a possible degeneration of the affected muscles, was described by Goutallier et al., 1989.10 Based on standardized preoperative CT scan images of the shoulder of patients undergoing rotator cuff surgery, he defined a rating system describing a muscular degeneration of torn rotator cuff units. Histologically the degeneration was shown to correspond to an infiltration of the muscular substance by fat, the so called fatty degeneration. The comparable observations were made in a rabbit model. 11
Another feature of the torn rotator cuff muscle, namely muscle atrophy, has been described on MRI by Nakagaki.12 Zanetti et al. demonstrated that the degree of atrophy measured on cross-sectional aeras of standardized para-sagittal MRI images inversly correlates to the degree of fatty degeneration.13
Both fatty degeneration and atrophy have been shown to be an irreversible process in the animal and in humans after successfull structural repair of the tendon.9 ,14,15 Furthermore, Gerber et al. demonstrated in a clinical study that degenerative muscular changes even may increase after repair suggesting that high tension resulting from reinsertion of a less elastic musculotendinous unit may worsen the state of degeneration of the affected muscle.9
Due to the irreversible loss of contractile properties of the repaired musculotendinous unit, weakness persists even after structural repair of the tendon. Furthermore advanced atrophy and fatty degeneration has been shown to be more often associated with retear when primary repair is attempted.9 ,16 Up to now no scientific data are available, defining precisely at which stage of muscular degeneration and in which part of the rotator cuff primary repair of a torn tendon is still possibly. However clinical experience suggests that in the presence of fatty degeneration higher than Grade II according to Goutallier, an alternative to primary tendon repair should be considered, especially if recovery of function and strength is the goal of treatment.
Those observations have fundamentally changed the way to evaluate and treat rotator cuff tears in the last years. A rotator cuff tear is no longer an isolated tendinous pathology, but much more a disease of the whole musculotendinous unit. This is of utmost importance when surgical treatment is considered.
Rotator cuff tears involving two tendons or more are defined as massive tears. They are commonly associated with muscle atrophy and fatty degeneration of the corresponding muscles, leading to decrease in contractile properties of the musculotendinous units. As advanced atrophy and fatty degeneration appears to be irreversible and often associated with retear when primary repair is attempted, such tears are considered irreparable.
In rare cases, the quality of the tendon is so poor, even in absence of advanced degenerative changes of the muscle, that secure repair to the bone is not possible. Such tears are encountered in revision surgery and are also considered irreparable.
Configurations of irreparable rotator cuff tears
Irreparable chronic rotator cuff tears can be divided into several patterns showing a different epidemiology, associated disability and natural history.
Because they are small and do not tend to retract, isolated supraspinatus tears can usually be repaired reliably. In rare cases fatty degeneration and atrophy of the supraspinatus muscle and/or poor tendon quality can render a tear irreparable. As the remaining parts of the cuff are intact, the functional deficit remains moderate. Pain and decrease in abduction strength are the leading symptoms.
Disruption of the infraspinatus is always associated with a supraspinatus tear and has been defined as posterior-superior tears. Per definitionem those tears are massive involving at least two tendons, the supraspinatus and infraspinatus, and may extend into the teres minor. In some patients where the tears extend inferior to the equator of the humeral head, the force couple between the anterior and posterior part of the cuff is disrupted. The required force to stabilize and to maintain a fixed fulcrum for rotation of the humeral head in the glenoid during flexion or abduction is insufficient. Functionally this leads to a superior migration of the humeral head and a decrease in abduction and flexion. Due to the insufficient infraspinatus, the strongest external rotator of the glenohumeral joint,17 those tears make movement of the hand to mouth or to the head difficult.
Isolated ruptures of the subscapularis are less frequent than supraspinatus or anteroposterior rotator cuff tears. Because unspecific complaints like pain and weakness without severe loss of function are in most cases the only clinical signs, subscapularis tears are underdiagnosed and treatment mostly occurs with delay. Then reconstruction of the tendon may not be possible anymore due to fatty degeneration and atrophy of the subscapularis muscle.
Anterosuperior tears are subscapularis tears involving the subscapularis and the supraspinatus tendons. They are even less common than isolated subscapularis tears and usually painful and disabling. Together with global tears, the represent a therapeutic challenge.
Tears involving both the anterosuperior and the posterosuperior portions of the rotator cuff are often associated with degenerative changes of the joint. They are defined as rotator cuff tear arthropathy.18 Painful limitation of motion is the leading symptom.
Irreparable rotator cuffs have not been considered as a single group in the evaluation of conservative treatment for rotator cuff tears. 19 ,20 Based on clinical experience, it appears that functionally compensated irreparable posterosuperior tears are relatively well tolerated. When pain becomes an issue, conservative therapy with strengthening of the intact portion of the rotator cuff may be an adequate therapeutic option, especially in the eldery patient. Irreparable subscapularis and anterosuperior tears are usually resistant to conservative treatment. However unremitting pain and/or decrease in function sometimes persit despite conservative treatment. Then surgery may be required. As mentioned above poor muscle and/or tendon quality does not allow reliable direct tendon to bone repair and alternative surgical techniques have to be considered.
Débridement and subacromial decompression
Arthroscopic débridement has been proposed for eldery patients whose main complaint is pain.21-24 This technique however, fails to restore strength. 21,23 Durability of pain relief has been questioned by some authors25,26, whereas others reported spectacular stable longterm results. 27
Allografts and synthetic cuff implants
The attempt to bridge large rotator cuff defects with rotator cuff allografts 28 or synthetic rotator cuff patches29 remained without reproducible results and never gained broad acceptance. Although the concept may appear very simple, it does obviously not solve the problem of the above discussed muscular disease in irreparable tears. For selected cases in which irreparability of the tear is caused by a specific tendon problem, tendon augmentation may be a suitable solution.
Rotator cuff advancement and transposition
To reduce tension at the side of repair, lateral advancement of the supraspinatus and infraspinatus musculotendinous units within the supra- and infraspinal fossa has been proposed to repair large rotator cuff tears.30 ,31 As for rotator cuff allografts or synthetics, this kind of procedure does not address the problem of degenerative changes encoutered in irreparable rotator cuff tears.
Local tranposition of the subscapularis tendon to repair large tears has been proposed by Cofield.32 As transposition seems to adversly affect active elevation, this technique has not found wide acceptance either.33
Fusion and resection arthroplasty
In patients with irreparable cuff tears, arthrodesis does not provide consistent pain relief.34 Neither does resection arthroplasty. 34 These techniques are used in selective cases and are considered as ultimate salvage procedures.
Constrained or unconstrained total shoulder replacement has been used in patients with irreparable rotator cuff tears. Due to eccentric loading early glenoid component loosening has been observed. 35-38
Although good pain relief has been reported with conventional or bipolar humeral head replacement, functional results are unpredictable, especially when severe loss of function is present prior to surgery. 39-44 1986, Grammont developed the so-called trumpet prosthesis to treat cuff tear arthropathies.45 The clinical experience with this implant have shown superior functional results compared to hemiarthroplasty or bipolar arthroplasty .46-49
Tendon transfer procedures
A tendon transfer is defined as a procedure in which the tendinous insertion of a muscle is divided and reinserted to a bony part or another tendon to supplement or substitute for the action of a nonfunctioning musculotendinous unit.17 The use of such procedures to treat irreparable rotator cuff tears is a relatively new field and the indications are not yet fully established. The clinical experience of several authors provides some guidelines and suggests that tendon transfer is a reliable option to treat symptomatic localized (anterosuperior or posterosuperior) irreparable rotator cuff tears in the abscence of articular changes of the glenohumeral joint.50-57
The exact nature of mechanical transduction within the skeletal muscle has not yet been fully elucidated. The understanding of the transformation of a neural signal into a force producing muscle contraction is based on two different stuctural models that have evolved over the years. The first type of models, based on Hill´s empirical work58,59, results in a higher-order nonlinear model considering three independent experimentally measured factors which describe the length-tension property, the force-velocity property and the dynamics of activation by neural imputs. The effects of simplifications characterizing such a model on the understanding of muscle behaviour during movement, have never been adequately quantified. However, eighth order Hill based antagonistic muscle-joint models have been used successfully to describe the biomechanics of complex joints, like the knee joint. 60
The second type of models is based on the structure and chemistry of muscle, describing excitation-contraction coupling and contraction dynamics.61 They result in complex partial differential equations.
The advantages and disadvantages of both types of models are still a matter of debate. For clinical purposes, it is important to have a capability to simulate human movement without modifying model parameters for different tasks. Brand´s approach to muscle properties is a simple and clinically useful synthesis of essential structural and biomechanical features of the skeletal muscle.62 Although the model does not consider ultrastuctural and biochemical features of human muscle, Brand was able to validate his concept in experimental and clinical work. Furthermore the data arising from Brand´s experiments on muscle properties, are considered as basic knowledge in tendon transfer surgery of the forearm and the hand.62
The sliding filament theory
Current understanding of the basic mechanisms of skeletal muscle contraction is based on the sliding filament theory proposed by Huxley.61 Muscle contraction occurs due to the interaction between actin and myosin muscle proteins, wich are arranged along with other structural and regulatory proteins in a regular pattern: the sarcomere.
Within each sarcomere, actin molecules form the thin filaments and the myosion molecules (thick filaments) carry regular spaced crossbridges along their length, giving a typical and constant pattern to all sarcomeres regardless which muscle is considered.
Sarcomeres are arranged in series to build myofibrils and to provide the needed muscle excursion. Furthermore the myofibrils are arranged in parallel to form the muscle bulk and generate the needed force.
The transduction of the neural signal into a contraction is a complex biochemical process, involving the release of acetylcholin at the neuromuscular junction, the calcium and ATP metabolisms within the cell and leading to a confomational change of the contractile filament and finally to a contraction. The details of the muscular contractile mechanism are still matter of debate.
Viscoelastic properties of skeletal muscle
The viscoelastic properties of a muscle arises from muscle contractile properties and the passive properties of non-contractile tissue within the muscle and are used to the describe the general relationship between muscle force and displacement and can be used to describe muscle behaviuour when either forces or displacement are imposed.
Although viscoelasticity is a term used to describe all aspects of muscle responses to mechanical disturbances, specific aspects have recieved more attention than others , probably due to their simplicity. Elasticity (or stiffness) describing the length-to-force relationsship and viscosity describing the velocity-to-force properties are the most relevant aspects for clinical applications.
The tension-length relationship
The amount of force generated by a muscle is known to depend upon its length. Indeed the sliding filament theory hypothetizes that changes in sarcomere length due to contraction or external loads result in different amounts of overlap between the thick filament containing crossbridges and the thin filament containing active binding sites.63-65 Although animal studies suggest that the length-tension properties of each sarcomere within a single muscle may vary, they are often similar enough to assume that the length-tension curve of the whole muscle has similar features. The generated force increases with length up to a certain point, remains on a plateau and then declines. (Fig.1)
The tension generated by the sarcomere reaches its maximum at the resting length. This maximum force will be maintained for even shorter lengths until the thin filament on the opposite ends of the sarcomere begin to overlap and interfere with force generation. Clinically the peak strength of an active muscle contraction will occur when the muscle is approximately in the middle of its total range between maximal stretch and full contraction.
If a muscle is streched to the point that there is no more overlap between the filaments, no force can be generated. On the other hand in the fully contracted sarcomere the overlap of the thin filaments will reduce force generation. (Fig.1)
|Figure 1: The tension-length curve|
The resting length (distance RL) is defined as the length of the sarcomere (or the muscle fiber) at which all the cross-bridges are within binding proximity to an actin active site.62 ,66 Clinically this occurs when the limb is in its resting and balanced condition.
An important feature is the relationship between the resting length (distance RL) of the sarcomere and its potental excursion (distance PE), defined as the distance between the stretched length and the fully contracted length. Brand et al. observed that RL and PE are approximatively of equal length.62 Therefore a sarcomere (or a muscle fiber) will be able to contract actively from maximal stretch to maximal contraction through a distance approximatively equal to its resting length. For example: a muscle fiber measuring 10 cm in the resting position could have an excursion of about 10 cm.
It is important to note that at each end of the excursion the generated tension would be almost zero. In addition the distance PE-50% indicates the potential excursion of a muscle within a range of at least 50% of its maximal tension.
The force-velocity relationship
The length-tension curve arises from an unatural status, in which the muscle length is experimentally determined and force is measured after supramaximal stimulation. In most activities, the muscle is either allowed to shorten or forced to lengthen by external loads. It has been recognized that the amount of force produced by the a given muscle for a fixed activation level depends on the speed the muscle length is changing and on the direction. In contrast to the length–tension relationship, which results primarily from the amount of overlap between the filaments, the force-velocity relationship arises primarily from cross-bridging cycling dynamics.
In the shortening muscle the maximum force is developed for zero velocity, i.e. for isometric conditions. As shortening velocity increases, the force drops in a hyperbolic fashion.58 The mechanisms acting to produce force-velocity properties during lengthening are basically different from those produced during shortening. Lengthening of a stimulated muscle is the result of an external load imposed which is greater than the force generated through contraction. The lengthening muscle always can generate more strength than the isometric contracting muscle. When the external load exceeds 1.2-1.8 times the maximal force generated in the isometric condition, the resistance against movement does not increase anymore. This phaenomenon is known as muscle yielding. At loads exceeding the elastic properties of the muscle fibers and the passive non-contractile elements, structural damage within the muscle tissue may occur.
It is important to note that the relationship described is based on measurements with maximally activated muscles under constant loading conditions. The shape of the force-velocity curve for the whole muscle in vivo has been shown to depend on the activation level and to dependent on previous movement history.67 ,68
Neural control of muscle contraction
All muscle properties required to maintain posture and produce movement are modified by change of the activation level which is initiated by the motoneuron group of the muscle. Although the process is complex and non-linear, it can be assumed that an increase in the activation level of single muscles leads to increases in stiffness and force. It is important to realize that control of limb posture, and movement is the result of a global and organized pattern of activation of all muscles involved. Therefore the transfer of a musculotendinous unit will require adaptation of the whole neural control system. This explains the long postoperative rehabilitation phase after structural healing of the transferred unit. Further description of the complex neural control mechanisms of limb mouvement and posture is beyond the scope of this work.
According to Brand, a muscle considered for transfer should match the dysfunctional recipient unit in regard of excursion, strength and orientation, so that its functionality can be restored. 62 ,69
The change in length - the exursion - that can be produced by a muscle is an important measure of its suitability for transfer. Indeed when a muscle is transferred, the required excursion in its new location may be different and may even be beyond the elasticity it can deliver.69
In vitro measurements
Assuming that all sarcomeres within a muscle are identical and that the resting length of one sarcomere is equal to its potential excursion, Brand postulated that the variable that characterizes a muscle must be be related to the sarcomere in series, the fiber length. He concluded that the average fiber length of a muscle is proportional to the potential excursion from the fully contracted to the fully streched muscle fiber. In fresh cadavers and under standardized conditions he measured the fiber length of the forearm muscles below the elbow and listed their potential excursion.62
In his comprehensive study Herzberg reported on the potential excursion of the main thirteen muscles of the shoulder girdle using Brand´s methodology.17
In vivo measurements
Freehafer et al. determined the potential excursion of the forearm muscles in vivo (so-called available excursion), considering both active and passive muscle properties.70 Their study showed a poor correlation between the potential excursion measured in vivo and the available exursion. Furthermore they found out that passive streching of a muscle only accounts for approximatively one third of the total available excursion of the tendon, while the remaining two thirds are resulting from active contraction. Finally they were able to show that soft tissue dissection around the muscle to be transferred significantly increases the available amplitude of the muscle. Although the durability of this effect is not known, it appears that surgical release around the transferred unit is warranted to increase the available excursion.
Another approach to determine muscle length in vivo has been proposed by Lieber et al.. In their study sarcomere length of flexor carpi ulnaris and extensor carpi radialis brevis muscles was measured intraoperatively using a laser diffraction device. The sarcomere length operating range varied between the muscles. Furthermore the sarcomere length of the flexor carpi ulnaris was different after transfer onto the extensor carpi brevis.
The available excursion of the shoulder muscles is not known.
Although a muscle can be strengthen by exercice or may show atrophy due to inactivity, the relative strength of a muscle within a given functional muscle group remains fairly constant .62
To determine the relative strength of the forearm muscles Brand used following equations:
rm = A x rv (1)
rc = rv/fl (2)
From equation (1) and (2) follows:
rc = rm / A x fl (3)
As the muscles can be weighted and the fiber length measured, the relative cross-section area of a group of muscles can be calculated by equation (3).
Furthermore, the cross-section of a muscle is proportional to the maximal tension it can generate.62 Therefore the relative tension of each muscle of a group of muscles can be calculated by equation (3).
In his study Herzberg analysed the relative strength of the shoulder girdle muscles.17
Force vector orientation
Estimation of the contribution of a muscle to the maximum isometric moment developed about a joint depends on severel accurate estimates; the muscle operating range on its tension-length curve, its physiological cross-section, and its moment arm. In tendon transfer surgery matching force vector orientation between the transferred and the dysfunctional muscle is a difficult task. The muscles available for transfer for a given dysfunctional musculotendinous unit are limited and their anatomical arrangement is usually very different from the muscle they should replace. Furthermore the moment arm of a transferred muscle may change and become less favourable because the position of the limb is changing. Clinical methods for analysis of the movement patterns after palsy or selective nerve blocks, movement analysis using electrophysiologic methods as well as calculation of moment arms in mechanical or geometric models has been proposed for a better understanding of the biomechanical effect of the muscles around the shoulder joint. However the kinematic of tendon transfer procedures around the shoulder has not yet been comprehensively described.
Restoration of muscle balance around the shoulder has been first described by L’Episcopo 1934.72 He did his research for a group of children with obstetrical plexus palsy and chose following tendon transfer procedure for treatment: the latissimus dorsi and teres major tendon were detached from the medial border of the humerus and reattached to its lateral border. This is changing the action of these muscles from internal rotators to external rotators. Consequently this is balancing the forces around the joint by weakening internal rotation strength and strenghtening external rotation strength.
The idea to perform a tendon transfer procedure for reconstruction an irreparable rotator cuff is attributed to Mikasa 1984.73 He proposed to transfer the trapezius for reconstruction of massive rotator cuff tears. The results of this technique remained unreproducible.
1985 a french group proposed the use of the anterolateral part of the deltoid for the reconstruction of irreparable lesion of the supraspinatus and infraspinatus.74 Although this technique is routinely used in Europe and is known to provide good pain relief, the deltoid flap transfer is not leading to recovery of strength and has not been used in North America. Furthermore, this transfer jeopardizes the integrity of the deltoid muscle, which may compromise the outcome of further procedures, like the implantation of a reversed total shoulder arthroplasty.
The first promising report on tendon transfer for irreparable tears of the superior-posterior rotator cuff was published 1988 by Gerber.75 He described the anatomical basis and first clinical results of the latissimus dorsi transfer for irreparable tears of the supraspinatus and infraspinatus and he presented overall outstanding longterm results 1992.50 Since then similar successful results with this method have been reported.51 ,52,76
The trapezius transfer has been proposed to compensate for irreparable tears of the subscapularis, but it has not become an established method.77 Wirth and Rockwood reported good and excellent results with the pectoralis major transfer for irreparable ruptures of the subscapularis.53 Vidil and Augereau reported successful transfer of the clavicular part of the pectoralis major for the same pathology, emphasing, however that weakness persisted despite improvement in function.78 In order to mimic the line of action of the subscapularis, Resch has proposed to reroute the tendon of the pectoralis major underneath the conjoined tendon54 and Warner has described the split pectoralis major procedure in which the clavicular part of the pectoralis major is rerouted underneath its sternal part.79
Only few studies have analysed the management of the irreparable subscapularis tendon tears. The original description of pectoralis major transfer by Wirth and Rochwood is considered as the gold standard for irreparable ruptures of the subscapularis.53 Although pain relief is usually achieved with this transfer, recovery of strength does not reliably occur with this procedure.53 ,57 From the biomechanical point of view the pectoralis major has an adequate excursion and strength. However the line of action of the pectoralis transfer may not be optimal because it is directed anteriorly to the coronal plane, whereas the subscapularis force vector points posteriorly.
In order to improve the line of action of the transfer, Resch et al. described a technique in which the upper part of the pectoralis major is rerouted underneath the conjoined tendon. In their opinion this would give a more favorable line of action for the transfer compared to the traditional technique. However major concern with this approach is the risk of injury of the musculocutaneous nerve, especially in the setting of failed prior surgery with extensive scarring. Although the complication rate with this version of the transfer is not higher than with the conventional transfer, short term results do not seem to be superior to those achieved by Wirth et al and Jost et al. In order to improve the line of action of the transferred pectoralis major without jeopardizing the musculoskeletal nerve, Warner proposed to reroute the sternal head of the pectoralis major underneath its intact clavicular head and to fix it to the greater tuberosity.55
In the following chapters new anatomical, biomechanical and clinical aspects relevant to the surgery of irreparable subscapularis tears are discussed. All chapters have the structure of scientific papers and some of them could already been submitted for publication.
Subscapularis tears should be released and repaired whenever the quality of the muscle allows primary repair. During surgery the neurovascular structures can be damaged. This may represent an anatomical constraint for circumferential release and direct repair. Intraoperative guidelines helping the surgeon to localize the subscapular nerves have not yet been clearly defined. Now the purpose of Chapter 2.1 is to describe the surgical anatomy of the subscapularis nerves and to define surgical guidelines.
The pectoralis major transfer is considered as the gold standard in the treatment of irreparable subscapularis tears. However the force vector orientation of this transfer may not be optimal in comparision to the situation at the subscapularis muscle. Anatomical studies suggest that the subscapularis muscle can be divided in two main components, the upper or thoracic part of the muscle respectively the lower or axillary part.80 Based on its location as well as on innervation and function, the teres major muscle turns out to be an optimal candidate for selective reconstruction of the lower part of the suscapularis muscle. It is the objective of Chapter 2.2 to describe the specific anatomy and surgical technique of the teres major tendon transfer for selective reconstruction of the lower subscapularis.
As discussed in the introduction, a muscle considered for transfer should match the dysfunctional recipient unit with respect to strength, excursion and orientation, in order to restore muscle balance and eventually function of the joint. Whereas potential excursion and relative tension of the shoulder girdle muscles are known17, detailled analysis of the vector orientation of the normal cuff and especially of common tendon transfer procedures around the shoulder have not yet been reported.
The purpose of Chapter 3.1 is to define a three dimensonal cadaveric model which allows vector calculation of the shoulder girdle muscles.
More specifically the purpose of Chapter 3.2 is to calculate the vector orientation of different transfer procedures for treatment of irreparable subscapularis tears using the model defined above. In addition, a comparision is made between the vectors of the transferred muscles and the original vector of the subscapularis musculotendinous unit.
Finally, based on the acquired anatomical and biomechanical data, Chapter 4.1 describes a new surgical concept for the treatment of irreparable subscapularis tears and reports on the early clinical experience in a series of 7 patients.
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