Proposed Answer

■ We will start by analysing the functional anatomy and relevant biomechanics of the shoulder girdle, especially during abduction. The shoulder is a very mobile joint and thus prone to instability. Stability is offered by:

- Static restraints mainly referring to the superior, middle and inferior glenohumeral ligaments. In addition, the coraco-acromial ligament also prevents superior subluxation of the humeral head

- Presence of an intact labrum and the vacuum effect from negative pressure inside the capsule. Thus, a capsular tear or a breach of the rotator interval (between the supraspinatus and subscapularis muscles) for instance will make the shoulder unstable, the concept that dislocation always involves a Bankart lesion is obsolete as was pointed out in the most recent literature. The relative articular mis match of the articulating surfaces is improved by the glenoid labrum. Thus, injury to the labrum as seen in superior labral anterior and posterior (SLAP) lesions in throwers by a peeling mechanism can also cause instability - The dynamic restraints, which can be viewed functionally as consisting of two groups of muscles: the rotator cuff muscles (supras-pinatus, infraspinatus, teres minor and subscapularis), and the scapula stabilisers (levator scapulae, rhomboids, serratus anterior, tra-pezius and pectoral muscles). Of the scapula stabilisers, the coordinated action of the trapezius, serratus anterior and levator are important in the necessary rotation needed to orientate the glenoid cavity for overhead sports

The act of shoulder abduction is usually initiated by the action of the supraspinatus, and then the main abduction force usually comes from the deltoid muscle. It is most important to note that proper abduction always involves rotation of the scapula; thus, there needs to be an intact rhythm of glenohumeral versus scapulothoracic movement and the ratio is usually 2:1. Without a stable base offered by the scapular stabilisers listed above, overhead activities will not be possible. This has been likened to a sea seal we see in television shows that tries to balance a ball on its nose. Another key point during abduction is the concerted effort of all four rotator cuff muscles plus the effect of the biceps long head, which acts as a humeral head depressor. It takes proper firing of all four rotator cuff muscles to centralise the humeral head inside the glenoid cavity, especially during the mid-abduction range. This is very important, as this so-called concavity-compression effect of the rotator cuff muscles will counteract the upward shear force of the deltoid. This is part of the key biomechanics involved in the act of abduction. Rotator cuff muscles act with force couples, and if one or more of the four muscles are not working, are torn, or weakened, the humeral head will not be exactly centralised in the glenoid cavity and impingement can occur in attempted overhead activity.

To understand impingement, which is essentially a clinical diagnosis, we must understand the local anatomy. Most people think that impingement is caused by one of the structures making up the supraspinatus outlet area namely: a hooked acromion, a thickened coraco-acromial ligament, osteoarthritis of the acromio-clavicular joint or even the distal clavicle. These cases do occur especially in the middle-aged patient (like ours) and can contribute to impingement pain and cuff tears and are referred to as "external impingement".

In the setting of overhead sports, a phenomenon called "internal impingement" can occur. The mechanism is due to repeated microtrauma and the hyper-angulation involved in overhead sports, causing rotator cuff weakness in particular and imbalance (e.g. repeated eccentric contraction of the supraspinatus, infraspinatus versus concomitant repeated concentric contraction of the subscapularis), with resultant element of shoulder instability and subluxation and causing internal posterosuper-ior glenoid impingement and even cuff tears despite the presence of a normal supraspinatus outlet (e.g. no hooked acromion, no osteoarthritis of the acromio-clavicular joint, etc.).

However, if one really wants to understand the pathogenesis of shoulder injury in tennis players, one needs to understand the biomechanics of the game of tennis itself, in addition knowing the basic biomechanics of the shoulder just discussed! One needs to know the following:

■ The adaptations of professional and amateur tennis players are very different

■ Difference between open stance versus square stance

■ Analysis of body movement constituting the forehand serve, the proneness to injury with the high angular velocities of these serves, the increased chance of impingement during the overhead action and the potential for muscle imbalance and microtrauma between the eccentric firing of the supraspinatus and infraspinatus, versus the concentric firing of the subscapularis, especially in follow-through

■ Analysis of the types of backhand serves, biomechanics involved and possible injuries

■ Knowledge of techniques for both professional and amateur players to prevent injury to the shoulder viz.:

- Physiological adaptations secondary to professional training and associated implications. A depression of the exercised shoulder is commonly found among highly trained professional athletes actively involved in overhead motions. This is caused by stretching of the shoulder elevating muscles, and generalised muscle hypertrophy of the racket-holding dominant upper extremity. There is reason to believe that this phenomenon further increases the chance of rotator cuff impingement, while the drooping of the shoulder itself may predispose to thoracic outlet syndrome. This same phenomenon, however, is not seen in amateur tennis players. Other changes in the dominant upper limb include significantly greater range of motion of external rotation of the shoulder relative to the non-dominant arm, while there is frequently a deficit of internal rotation of the dominant arm. Prolonged microtrauma to the cuff frequently leaves the athlete with an overall deficit of the total range of motion; however, the overall demands of the game of tennis on the shoulder is such that it requires concentric work to stabilise the shoulder, with effective depression of the humeral head to prevent shoulder impingement. The added demands of forehand and back hand serves will now be discussed

- Open stance versus square stance technique: Comparisons have previously been made using a three-segment rigid body model in order to calculate the kinetics of the wrist, elbow and shoulder joints at the moment of impact. It was found that the open stance created lower resultant velocity of the racket at the moment of impact compared with the square stance, for professional tennis players and experienced amateurs. The largest component of the resultant joint torque was being generated by horizontal shoulder adductors, followed closely by varus torques on the elbow, and shoulder internal rotation torques. In addition, there was significantly greater peak shoulder internal rotation torques associated with the square stance than with the open stance. Overall, the peak upper extremity torques recorded were not dissimilar to those recorded for professional basketball pitchers and are of a magnitude that may well contribute to overuse injuries

- The tennis forehand and implications of high torque: it is common knowledge that the tennis forehand serve can produce enormously high angular velocity of the shoulder joint. Analysis using three-dimensional motion analysis revealed that these very high velocity tennis serves can be broken down and thought of as being made up of different segmental motions, including: trunk tilt, upper torso rotation, pelvis rotation, elbow extension, wrist flexion, and finally shoulder internal rotation during the final motion segment, which eventually involves sudden deceleration forces that can also harm the posterior structures of the shoulder. In addition, shoulder internal rotation torque was found to be greater in gener al for males, resulting in higher angular velocities attained relative to female professional athletes as expected

- The tennis backhand, and single versus double backhand: results of motion analysis of the single tennis backhand reveal the need for coordinated motion of the following body segments: hips, shoulder, upper arm, forearm, and hand/racket rotations. The same body segments are involved in the double backhand serve. Professional tennis players frequently produce comparable tennis racket horizontal velocities with either the double or single backhand techniques. The single backhand technique features a more rotated shoulder alignment than the double backhand technique upon completion of the backhand swing. During ball impact, the point of impact tends to be further in front, thus producing a slightly higher torque in the single backhand serve than the double backhand technique. The fact that the performance of professional players using the double backhand is comparable to the single backhand technique stems from the fact that players with the double backhand technique tend to delay the horizontal acceleration of the racket towards the ball, and thus are capable of comparable hitting motion at the time of ball impact

- Some coaching techniques for injury prevention: firstly, the motion analysis of professional athletes mentioned above can be recorded and played back to keen tennis players concerning the correct technique to adopt. However, other points worthy of note in injury prevention include: attention to the adequacy of knee bending since manoeuvres such as these are needed to exercise proper force transfer from the lower limbs through the arms during the tennis serve. If not performed properly, this will increase the demands on the shoulder to create the service speed needed and thus increase the chance of injury to the shoulder. Also, seemingly simple manoeuvres like ball tossing are in fact important, since if poorly done, this can result in excess spinal extension, encouraging shoulder impingement. Likewise, the likelihood of shoulder injury is increased if there is poor court positioning, which potentially can increase ground-stroke action and increase stress to the shoulder joint.

Analysis and differential diagnoses of pain in the shoulder in tennis players will now be discussed with regard to our patient present with pain. Owing to the complex anatomy of the shoulder complex that consists of four joints: sternoclavicular, acromio-clavicular, glenohumeral, and scapulothoracic articulations, analysis of pain arising from the shoulder girdle is not always straightforward. To complicate matters, we need to be mindful of the fact that pain can also be referred from the cervical spine, and other rarer causes like thoracic outlet syndrome, which was discussed previously. Previous literature analysis on shoulder pain in tennis players showed that in fact there are two most common causes that produce shoulder pain, namely the impingement syndrome and glenohumeral instability.

Tennis players with impingement syndrome typically present with pain during overhead strokes and serves. Bedside Neer's and Hawkin's test, we can consider that the use of the impingement test involving almost immediate pain relief on injection of anaesthetic to the subacro-mial area is often diagnostic if the diagnosis is unsure. Tennis players with instability present with pain and a sensation of the shoulder "slipping". Dislocation is rare. Most cases turn out to be in fact a form of internal impingement caused by fatigue of the rotator cuff muscles, particularly the supraspinatus and infraspinatus, thus causing difficulty in centralising the humeral head in position during overhead activity. An associated disruption of the normal ration of glenohumeral to scapulothoracic movement is not uncommon, causing further loss of sports performance, poor serve velocity, and easy fatigability.

Finally, some "external" factors that can contribute to the shoulder pain of our middle-aged patient who is involved in computer work potentially include:

■ Technique and training errors: these have just been discussed above, including the methods of prevention of injury

■ Improper equipment: notice that the game of tennis involves open kinetic chain activity of the upper limb. This, together with the high velocities encountered, can cause injury to the whole mobile kinetic chain, not just the shoulder. Thus, the use of improper equipment, like improper size of the racket, will increase the chance of injury to, say, the wrist and elbow as well. Other possible external factors that are sports-related include the playing surface, environment warm-up exercises, etc.

■ An example of an external factor that is job related is that by the nature of his job as a computer worker, he frequently protracts his sca pula during work, and lack of exercise to the other scapular stabilisers that are essential for overhead sports can predispose to an abnormal glenohumeral to scapulothoracic rhythm

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