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Three-dimensional Glenohumeral Arthrokinematics In Vivo: A Review and Synthesis of the Literature Brad McIntosh BSc (Ex Sci), MPT, DPT Sydney Sports & Orthopaedic Physiotherapy, Sydney, Australia ABSTRACT A significant body of evidence has accumulated attempting to describe the kinematics of the glenohumeral articulation. This has made it difficult for the clinician and student to synthesise and understand, and may have led to the development of simplistic models of joint motion that are incorrect or incomplete. Much of our early understanding came from plain radiographic studies and simulations using cadaveric models, which inevitably introduce estimations and error into the calculation of shoulder joint movement. More recently, advances in imaging technology have allowed us to assess three-dimensional arthrokinematics in vivo, which may enable a more accurate understanding of the function of the shoulder joint. This review will consolidate the current research on three-dimensional glenohumeral kinematics, considering electromagnetic tracking, computed tomography (CT), and magnetic resonance imaging (MRI) studies. The impact of this knowledge on our current use of the concave-convex rule in the glenohumeral joint will then be discussed. INTRODUCTION A large amount of information is now available describing movement at the glenohumeral articulation, and simplistic arthrokinematic models are beginning to be questioned. In a recently published review, Brandt et al.1 examined the evidence for and against Kaltenborn’s convexconcave rule as it applies to the shoulder joint. They concluded that this indirect method of determining the direction of humeral head translation needs to be reconsidered. Research on the mechanics of the shoulder joint has progressed well beyond simplistic models, and it has become quite clear that shoulder arthrokinematics are far more complex and individualised. Much of our recent understanding has come from kinematic modelling studies using cadaver shoulders. In vitro modelling allows a very accurate assessment of humeral head translation, however, it introduces the potential for error in estimating the effects of muscle function and simplifies the normal dynamic interactions of the shoulder joint. Plain radiographic studies, ultrasound, conventional-CT, and conventional-MRI are limited to two-dimensional analysis, and may neglect movements that are not in the imaging plane. Secondly, reproducibility is significantly limited due to the inevitable change in the imaging plane in subsequent sessions. Plain radiography introduces the additional limitation of projection artifacts, while traditional CT and MRI studies are also limited by not allowing imaging in clinically relevant arm positions. Recent advancements in scanners designed for interventional MRI and CT procedures have addressed a number of these problems. Open scanners allow the shoulder to be positioned functionally in abduction and external rotation, and assessed under conditions of muscle activity. Three-dimensional digital post-processing techniques can then be used to assess humeral head positioning and translation relative to the glenoid. The reliability of electromagnetic tracking devices has improved, and can also be used to determine glenohumeral translation in vivo. As such, the literature on glenohumeral arthrokinematics using CT, MRI, and electromagnetic tracking will be presented in this review. METHODS A literature review was conducted using Medline, and was restricted to English language articles. The search words included: arthrokinematics, kinematics, joint mechanics, shoulder, and glenohumeral. Each title and/or abstract was then searched for a potentially suitable focus on arthrokinematics in normal or injured shoulders. The reference lists of the identified articles were also manually searched using the same criteria. Ten articles were reviewed and included in the analysis and synthesis. These included 1 study using electromagnetic tracking, 1 study using CT, and 8 utilising MRI. The literature was grouped and analysed according to the imaging modality used, with specific focus on the anteroposterior and superoinferior translation of the humeral head on the glenoid. RESULTS Magnetic Resonance Imaging (Table 1) Rhoad et al.2 used conventional MR imaging with 3-D post-processing to assess the glenohumeral translation from neutral to maximal internal and external rotation at 0 of abduction. They calculated the error of their measurements by comparing the imaging technique to translational measurements from cadaver shoulders. The quantitative analysis revealed a standard error of the mean of 0.11mm, and they concluded that this was an accurate, noninvasive technique for assessing shoulder kinematics. Their assessment of 9 asymptomatic shoulders demonstrated a mean posterior translation of 0.64mm from neutral to maximal internal rotation, and 3.02mm of anterior translation from neutral to maximal external rotation. This was a preliminary study of a new technique for assessing glenohumeral arthrokinematics, and as such there were a number of limitations. The use of a conventional scanner in this study meant that images were restricted to the non-functional position of 0 abduction. In addition, only the translational values were reported, making it difficult to compare the positional results of this study to other kinematic studies. The study did manage to successfully demonstrate the accuracy of MR imaging and 3-D post-processing. Table 1. Summary of Reviewed Studies Authors Subjects Starting Position Rhoad et al.2 (MRI) 9 asymptomatic shoulders Not Reported Graichen et al.3 (MRI) 15 asymptomatic shoulders Graichen et al.4 (MRI) Anterior-posterior Translationa Superior-inferior Translationb 1.58mm SUP & 1.55mm ANT at 30 ABD -0.64mm (neutral to max IR) +3.02mm (neutral to max ER) (measurements taken at 0 ABD) +0.83mm (30-90 ABD) -2.45mm (90-150 ABD) -0.05mm translation (neutral to max IR) +0.67mm translation (neutral to max ER) (measurements taken at 0 ABD) -1.22mm (30-150 ABD) 12 asymptomatic shoulders 0.6mm SUP & 1.3mm POST at 30 ABD (ABD muscle activity) 0.2mm SUP & 0.5mm ANT at 30 ABD (ADD muscle activity) -1.0mm (30-150) ABD muscle activity -1.1mm (30-150) ADD muscle activity +1.2mm (30-120 ABD) & -0.9mm (120-150) ABD muscle activity. 0mm (30-150) ADD muscle activity Beaulieu et al.5 (MRI) 10 asymptomatic shoulders 1.0mm SUP at 0 ABD Not Reported - 1mm (0-120 ABD) Hodge et al.6 (MRI) 11 symptomatic shoulders & 8 contralateral control shoulders Both shoulders of 12 patients with traumatic & 10 with atraumatic instability Not Reported Not Reported Small, non-significant SUP translation from 25-100 of ABD Positions at 30 ABD: Control = <1mm off-centre Traumatic = <1mm off-centre Atraumatic = sig. off-centred, either posteroinferior or anterosuperior Measured from 30 - 90 ABD + full ER Control = +0.2mm Traumatic = +3.0mm Atraumatic = -1.9mm Measured from 30 - 90 ABD + full ER Control = -1.2mm Traumatic = -1.7mm Atraumatic = -1.8mm Von Eisenhart-Rothe et al.8 (MRI) 28 healthy shoulders, & 14 patients with atraumatic instability Positions at 30 ABD: Control = 0.4mm POST & 0.2mm SUP Atraumatic = 0.6mm POST & 0.4mm SUP (large interindividual variations noted in this group) Measured from 30 - 90 ABD + full ER Control = +1.2mm Atraumatic = -0.7mm Measured from 30 - 90 ABD + full ER Control = -0.3mm Atraumatic = -2.7mm Schiffern et al.9 (MRI) 6 subjects for 9 asymptomatic shoulders 3 shoulders with minor ANT instability; 3 control shoulders 12 shoulders; 6 with posterior & 6 with anterior tightness Measured at 35 ABD from 0 - 60 ER -3.1mm Measured at 35 ABD from 0 15 IR +0.9mm -1.6mm (90 ABD/ER – full cocking) ANT instability -4.6mm (90 ABD/ER – full cocking) – Normal shoulders -3.0mm (0-120 ABD) – POST tightness -5.2mm (0-120 ABD) – ANT tightness Not Reported Baeyens et al.12 (CT) Positions at 35 ABD: 7/9 shoulders exactly centred, 1 shoulder 1.1mm ANT, & 1 shoulder 1.0mm POST Not Reported Von Eisenhart-Rothe et al.7 (MRI) Lin et al.15 (Electromagnetic Tracking) a + = Anterior, - = Posterior, b Not Reported -0.3mm (90 ABD/ER – full cocking) – ANT instability +0.18mm (90 ABD/ER – full cocking) – Normal shoulders +4.9mm (0-120 ABD) – POST tightness -0.8mm (0-120 ABD) – ANT tightness + = Superior, - = Inferior, ANT = anterior, POST = posterior, SUP = superior, INF = inferior, ABD = abduction, ADD = adduction Influence of Muscle Activity Not Reported POST translation of 1.22mm (90 ABD) ANT translation of 0.37mm (120 ABD) INF translation of 0.8mm (90 ABD) & 1.0mm (120 ABD) Humeral head significantly more SUP at 90 & 120 under ABD muscle activity, compared to ADD muscle activity Humeral head significantly more POST at 60, 90, 120 under ABD muscle activity, compared to ADD muscle activity All measurements were active and therefore under the influence of muscle activity All measurements were active and therefore under the influence of muscle activity Control and traumatic instability shoulders became significantly more centred under muscle activity Atraumatic instability shoulders demonstrated sig. ANT translation under muscle activity. There was no centralisation Centring of the humeral head was noted in the normal shoulders The humeral head was still significantly de-centred in the atraumatic group in both the ANT-POST and SUP-INF directions (i.e. greater than 2 times the SD in the healthy group) Not Reported Not Reported All measurements performed with active range of motion In another early 3-D MRI study, Graichen et al.3 used an open MR scanner to assess arthrokinematics in 15 healthy shoulders. They examined the arm at five different angles of abduction from 30 to 150, with the arm being positioned passively. The influence of muscle activity was then assessed by adding a 1kg weight with an adducting force during imaging. Three-dimensional image reconstruction was used to calculate the centre of mass of the glenoid articular surface and the midpoint of the humeral head. The authors were able to demonstrate the centring effects of muscle activity, reporting significant inferior and posterior translations in most positions with the addition of a 1kg weight. They used a different technique to determine the mid-point of the humeral head, applying a virtual reality matching technique to obtain a spherical representation. They demonstrated the accuracy of this technique by assessing its reproducibility, however, the absolute error of the technique was not assessed. The use of an open MR scanner did, however, allow for assessment in more functional arm positions. In a more recent study, Graichen et al.4 again assessed the effects of abducting and adducting muscle activity in 12 healthy subjects aged between 21 and 35 years. Images were obtained at five different angles of elevation (30-150) in neutral rotation, with the subject positioned supine on a pad that allowed free scapular movement. A force of 15 N was applied in either an abducting or adducting direction. Similar imaging and post-processing techniques were used to their previous studies. Abducting muscle activity resulted in a significantly more superior humeral head position at 90 and 120 of abduction, when compared to adducting muscle activity. Abducting muscle activity also led to a significantly more posterior position of the humeral head from 60-120 of abduction. They concluded that adducting muscle activity resulted in a significant anterior and inferior translation, and suggested that this has implications in the successful management of impingement syndrome. This is a neat conclusion from their data, however, all subjects were asymptomatic at the time of the study and generalisation of the results may be questionable. Another technique using an open MRI system was utilised by Beaulieu et al.5 They assessed 10 asymptomatic shoulders, using images acquired at a rate of 2.5 seconds per image. This dynamic measurement allowed assessment under more functional movement conditions, however, the image quality was significantly reduced. Only 2-D analysis was possible with this technique, so movement in planes other than that imaged was not detectable. Only the values for superoinferior glenohumeral translation were reported from 0-120 of abduction. During abduction and adduction, the results showed that the humeral head remained centred on the glenoid. Deviations from the glenoid centre point were within the range of the standard deviation of the measurements. In general, the average humeral head position was between 1.5mm inferior and 2mm superior to the glenoid centre point throughout the range of movement. Utilising the same approach to image acquisition and measurement, Hodge et al.6 reported on 11 symptomatic shoulders. Each subject was experiencing unilateral shoulder pain or instability that required surgical management. Specific shoulder pathology was not detailed, and as such, the symptomatic shoulders may have included a number of subgroups. Their results also demonstrated centring of the humeral head on the glenoid in all but two of the symptomatic shoulders, which showed large superior subluxations throughout the range of abduction. Again, no consideration was given to anteroposterior translation. Von Eisenhart-Rothe et al.7 investigated the 3-D translation of the humeral head in 12 patients with traumatic anterior instability and 10 patients with atraumatic instability. Comparison was made with the contralateral shoulders in each of the groups. Images were obtained at 30 and 90 with no rotation, and at 90 with maximal external rotation. The influence of muscle activity was assessed by introducing a weight of 10N to the 90 abduction and external rotation condition, producing a constant contraction of the internal rotators. Images were obtained using a technique identical to the study by Graichen et al.4, with an open-MRI scanner and 3-D postprocessing. The starting position of the humeral head was assessed at 30 abduction and neutral rotation. In the contralateral shoulder of the traumatic instability group, which could be considered a “control” shoulder, the humeral head was centred on the glenoid within 1mm of the centre point. In the unaffected shoulder of the atraumatic instability group, the humeral head was significantly more superior and posterior. The affected shoulder of the atraumatic instability group showed wide variation. When absolute values were considered, these shoulders were approximately 2mm more off-centre in both planes than the normal shoulders. In some cases this involved an anterosuperior direction, while others demonstrated a posteroinferior displacement. In the normal shoulders, anteroinferior translation occurred from 30 to 90 of elevation. From this position to that of full external rotation, all normal shoulders translated inferiorly, while half moved anteriorly and half moved posteriorly. With muscle activity in this position, all normal shoulders became more centred. In comparison, the contralateral shoulders of the atraumatic instability group translated superiorly and anteriorly with elevation. During external rotation, the direction of translation was highly variable between individuals. Under the influence of isometric muscle activity, all shoulders in this group translated anterosuperiorly, and thus no centralising effect was found. In the involved shoulders of patients with traumatic instability, increased anteroinferior translation was demonstrated from 30 to 90 of elevation when compared with the normal shoulders. When external rotation was introduced, the anteroinferior translation was twice as great as that in the normal shoulders. The humeral head became more centred when isometric muscle activity was introduced. In the affected shoulder of patients with atraumatic instability, translations were also variable. Patients generally demonstrated anterosuperior or posteroinferior migration of the humeral head. The absolute translations were significantly greater in the anteroposterior direction in this group than the normal shoulders. With muscle activity, there was a significant anterior translation in all cases, and therefore no centralising effect. A B C Starting position 90 + external rotation Figure 1. Representative examples of the humeral head starting position and position at 90 abduction and external rotation for the (A) control, (B) atraumatic, and (C) traumatic instability groups. The authors were therefore able to demonstrate significant differences in humeral head position and translation between normal shoulders and shoulders with traumatic and atraumatic instability. The direction of translation was not uniform in patients with atraumatic instability, in either the affected or unaffected shoulders. The absolute amount of translation was approximately 3 times higher in the unaffected shoulders and 5 times higher in the affected shoulders of this group when compared to the normal shoulders. Additionally, muscle activity showed no centralising effect in the shoulders of patients with atraumatic instability. In a similar study also published by Von Eisenhart-Rothe et al.,8 an almost identical design was used to assess the shoulders of 28 healthy individuals and 14 individuals with atraumatic instability. Additionally, they considered the influence of scapular positioning on the decentring of the humeral head. There was a high correlation (r = 0.60-0.87) between the scapular and humeral head positions during passive movement, suggesting that alterations of scapular motion may be important in the decentring of the humeral head. The unstable shoulders demonstrated significant differences in the humeral head position in various degrees of elevation and external rotation. The mean values reported in Table 1 should be interpreted with caution due to the large inter-individual variation noted. Non-uniformity in the direction of the instability created much more centred mean values for the group than were noted in the individual measurements. Four of the five patients with anteroinferior instability demonstrated a superior starting position (at 30 abduction) and translated significantly more in the anteroinferior direction during abduction and external rotation. Eight of the nine patients with posteroinferior instability demonstrated a significantly greater posteroinferior position throughout all of the measurements. As in the previously reported study, muscle activity did not result in significant centring of the humeral head in the atraumatic group, whereas the normal group demonstrated a centring effect. Schiffern et al.9 considered the position of the humeral head in relation to the glenoid in midrange positions. They assessed nine asymptomatic shoulders using open-MRI and reported the anteroposterior translation during passive rotation at 35 of abduction in the scapular plane. This position was chosen as it was considered to approximate the position at which the capsule was most lax. Images were taken at 15 increments of rotation from 15 of internal rotation to 60 of external rotation. The centre-point was determined manually using templates that matched the circumference of the humeral head. The glenoid centre was determined by drawing a line between the anterior and posterior margins of the bony glenoid, and constructing a second, perpendicular, bisecting line. The aim of this study was to demonstrate that in mid-range positions (0 - 45 of external rotation at 35 of abduction), the humeral head would be centred on the glenoid without voluntary muscle activity. The authors considered this to be the case after presenting their results, which demonstrated a mean of only 0.1mm of anterior translation in this range. When considering the 60 external rotation position, which the authors considered to be more of an end-range position, the mean translation was 3.1mm in a posterior direction. The authors consider this to be support for the concavity-compression effect, which suggests that resting muscle tone may be enough in a normal joint to maintain humeral head centring in mid-range positions. The study was limited by the use of low-field MRI with limited resolution, 2D measurements, and a small sample size of only 6 participants. A number of studies have assessed the width of the subacromial space in asymptomatic shoulders and shoulders of patients with impingement. While these studies have not presented specific data on translation of the humeral head on the glenoid, the superoinferior translation can be estimated. Graichen et al.10 assessed the images of 10 subjects with impingement syndrome and 10 healthy volunteers. Open-MRI was used to obtain images at 60, 90, and 120 of abduction under relaxed and active conditions, and 3-D post-processing was used to calculate the minimum acromiohumeral distance. In the asymptomatic shoulders, the mean minimum acromiohumeral distance decreased significantly from 90 to 120 of passive abduction, as the greater tuberosity approached the acromion. Under conditions of muscle activity, the acromiohumeral distance was significantly decreased at 60 of abduction and significantly increased at 120 of abduction. This meant that the mean minimum acromiohumeral distance remained constant throughout the three measurements under conditions of muscle activity. There was no difference between the healthy shoulders and shoulders with impingement under passive conditions. However, a significant reduction in space was demonstrated in the impingement group at 90 of abduction under active conditions. The authors therefore concluded that abnormal activity of the shoulder muscles may play an important role in the pathogenesis of impingement syndrome. They also suggest that functional imaging under conditions of muscle activity, with comparison to the unaffected side, has the ability to objectively identify impingement syndrome. Two further studies conducted by the same group have used a similar technique to assess the influence of abducting and adducting muscle activity on the subacromial space width.4,11 Both studies reported a significant increase in the subacromial space with adducting muscle activity as compared to abducting muscle activity in all positions of abduction. The authors conclude that harnessing the effect of the adductor muscles may enable a depression of the humeral head and assist in the conservative management of impingement syndrome. Computed Tomography (Table 1) Three-dimensional reconstructions of helical CT data were used by Baeyens et al.12 to determine glenohumeral arthrokinematics in 1st-division European handball players. Three symptomatic shoulders were identified as meeting the criteria for minor anterior instability, and three asymptomatic age-matched controls were recruited. The symptomatic shoulders were required to demonstrate dysfunction at the late-cocking phase of throwing, no signs of impingement, and negative anterior instability tests. Images were obtained at 90 abduction/90 external rotation, and compared to 90 abduction/full external rotation/full horizontal extension. The data for the asymptomatic control group was also presented in a separate study by Baeyens et al.13 The translation from position 1 to position 2 resulted in a significantly more posterior position of the humeral head centre in the asymptomatic shoulders. The reported amount of posterior translation was also greater in this group. The authors suggest that in this end-of-range functional position, shoulders with minor anterior instability may demonstrate dysfunction of the anterior part of the inferior glenohumeral ligament complex. The results clearly suggest that in normal shoulders, the humeral head tends to move posteriorly in the late-cocking phase of the throwing motion. The implications of the study were significantly limited by the very small sample size. Only 3 shoulders were identified that matched the criteria for minor anterior instability as defined by the authors. This study, and the separate presentation of the asymptomatic group data13, were the only studies identified that used CT imaging and 3-D post-processing to determine glenohumeral arthrokinematics. Electromagnetic Tracking (Table 1) The reliability of in vivo measurement of glenohumeral translation using electromagnetic tracking is limited. In a study of the Flock of BirdsTM electromagnetic tracking device by Stokdijk et al.,14 10 repeated measurements of flexion, scapular plane elevation, and abduction were conducted on one subject. The standard deviation of the measurement of the joint rotation centre was reported to be 3.5mm. The authors concluded that this was a reliable determination the joint rotation centre. However, when considering the small translations of the humeral head during shoulder movements, as determined by MRI, this value may be considered too high. In comparison, Graichen et al.3 reported an error of 0.017mm using open-MRI and a computer sphere-matching model for determining the centre of rotation. Only one study using electromagnetic tracking was identified that presented glenohumeral translation data during active range of motion. Lin et al.15 attempted to offset potential problems with using the electromagnetic tracking device for in vivo measurement by selecting subjects with low body mass indices and limiting range of motion to 120. These limitations were designed to minimise error from soft-tissue movement over the bony landmarks, and to minimise scapular movement. Twelve subjects performed active flexion, abduction in the frontal plane, and elevation in the scapular plane, with movement being recorded by the FASTRAK 3-D electromagnetic motion-capturing system. Six subjects were deemed to have posterior shoulder tightness and 6 subjects had anterior shoulder tightness. The humeral head centre was then estimated using a helical axis method. Both groups demonstrated posterior humeral head translation during each of the active movements performed. The group with anterior shoulder tightness demonstrated significantly greater posterior translation in all movements. The group with posterior tightness showed superior translations in all measurements, while the anteriorly tight group demonstrated inferior translation with both abduction and flexion. As the authors state in their discussion, however, the limitations in the accuracy of these in vivo measurements means the results should be interpreted with caution. DISCUSSION The convex-concave rule as described by Kaltenborn16 in 1980, Kaltenborn and Evjenth17 in 1989, and first presented by MacConaill18 in 1953, has had a significant impact on the assessment and treatment of joint dysfunction by manual therapists. The rule predicts the translational movement of the mobile segment—relative to the fixed segment—based on the geometry of the joint surfaces. In the glenohumeral joint, the convex humeral head and concave glenoid surfaces dictate the direction of “roll” and “glide”. Stated simply, when the mobile humeral head (convex) moves on the fixed glenoid (concave), the roll and glide are in opposite directions (Figure 2). For example, abduction would be accompanied by a caudal glide of the humeral head while extension and external rotation would see the humeral head glide anteriorly. Roll (Superior) Humeral Head Glenoid Glide (Inferior) Figure 2. The convex-concave rule of roll and glide as it applies to abduction of the glenohumeral joint. The in vivo studies assessed in this review demonstrate some disagreement with the expected direction of glide determined by the convex-concave rule in the shoulder. For example, Schiffern et al.9 demonstrated a 3.1mm posterior glide while externally rotating the shoulder in 9 asymptomatic subjects. Similarly, Baeyens et al.12 demonstrated a 4.6mm posterior translation in 3 asymptomatic shoulders, with the arm in the full cocking position of 90 abduction, horizontal extension, and full external rotation. In contrast, the convex-concave rule would suggest that these positions of external rotation should lead to an anterior translation of the humeral head. This contradiction may be explained by the interaction between the active and passive control systems of the glenohumeral joint, namely the rotator cuff musculature and the capsuloligamentous stabilisers1. A number of in vitro studies have also presented data that questions the validity of the convexconcave rule in the shoulder. Brandt et al.1 have recently described the in vitro research as part of their review of the validity of Kaltenborn’s rule. The authors’ conclusion is that the convexconcave rule should not be accepted in all cases, and that clinical decisions regarding the appropriate assessment and treatment of shoulder dysfunction should be made carefully until further good quality research is conducted. While in vitro research may estimate the role of the dynamic stabilisers of the joint, there may be significant error in trying to reproduce these forces in cadaver modelling. By studying arthrokinematics in vivo, it is possible to simultaneously assess the influence of both the active and passive control systems. The position of the humeral head appears to be strongly influenced by the active control system, both in the mid-range and at the extremes of motion. When assessing the influence of muscle activity on the position of the humeral head, Graichen et al.3 demonstrated a centring effect in normal shoulders in the mid-range of motion from 30-150 of abduction. Von Eisenhart-Rothe et al.7,8 also demonstrated this active centring effect at the endof-range, demonstrating a centred humeral head at 90 abduction and full external rotation in normal shoulders under conditions of muscle activity. It appears that in the active asymptomatic shoulder, the humeral head remains centred throughout the range of motion, both in the anteroposterior and superoinferior directions. This phenomenon is not seen in shoulders with atraumatic instability. Von Eisenhart-Rothe et al.7 demonstrated an anterior translation under conditions of muscle activity in shoulders with atraumatic instability. This led to a humeral head position that was significantly off-centred. In another study by the same group of authors, the atraumatic instability group was again significantly off-centred under conditions of muscle activity.8 The group mean was greater than two times the standard deviation of the asymptomatic group in both the anteroposterior and superoinferior directions. These data suggest that treatment directed at the capsulo-ligamentous stabilisers in this group is likely to lead to a poorer outcome, and the results of surgical stabilisation for atraumatic instability support this. For example, Joseph et al.19 evaluated the outcomes of laser capsulorraphy in multidirectional instability and found an overall recurrence rate of 40%. The research presented in this review allows us to examine the kinematics of the normal glenohumeral joint in vivo, under both passive and active conditions. Different research techniques and reporting make it difficult to combine data from individual studies, however, basic generalisations on the direction of translation can be made (Table 2). Investigating passive abduction demonstrates some common themes. During the first 90, the humeral head translates anteriorly and inferiorly on the glenoid. Above 90, there is a larger posterior translation, resulting in an overall posterior translation throughout the range. The inferior translation up to 90 continues throughout the remainder of the range, resulting in an overall inferior translation. When considering active range of motion, the humeral head remains centred on the glenoid in both the anteroposterior and superoinferior directions. Table 2. Number of studies reporting the direction of humeral head translation during abduction in asymptomatic shoulders. Passive Active Anterior Posterior Centred Superior 0-90 90-150 0-150 0-90 90-150 0-150 Inferior Centred The importance of muscle activity on controlling arthrokinematics is not only evident in the normal shoulder, but has also been identified in shoulders with impingement syndrome. Graichen et al.10 demonstrated no significant differences during passive motion, however, active motion demonstrated a marked reduction of acromiohumeral distance in the impingement group. The two factors most likely to influence the acromiohumeral distance are alterations in scapular motion and superior translation of the humeral head on the glenoid. In a later study by the same group,4 a significant superior translation of the humeral head was found during active abduction. It can be stated, that the active control system plays a key role in determining kinematics of the glenohumeral joint in normal shoulders, shoulders with atraumatic instability, and shoulders with impingement syndrome. The starting position of the humeral head is another important consideration (Table 1). It has been postulated by Magarey & Jones20 and J. McConnell (oral communication, Spetember 2006) that an abnormal starting position may predispose the shoulder to movement dysfunction, and the reviewed literature may provide some support for this hypothesis. It is clear that the normal starting position of the humeral head is directly centred on the glenoid in both the anteroposterior and superoinferior directions, with 5 out of 6 studies demonstrating a deviation of less than 1mm.3,4,5,7,8,9 Two studies also assessed the starting position of shoulders with atraumatic instability and found them to be significantly off-centred by at least two times the standard deviation of the normal shoulders.7,8 The direction was, however, not uniform, resulting in the reported group mean being almost centred. The studies that assessed the influence of muscle control on humeral head positioning suggest that an abnormal starting position may signal dysfunction in the active control system.3,7,8 Further specific research is required to identify the existence and importance of this abnormality in various subgroups of patients with shoulder dysfunction. Despite the advances currently being made in in vivo studies of shoulder arthrokinematics, the results of the reviewed studies need to be interpreted with caution. Individual variations may lead to a small average translation, or a centred average position, when in actual fact the absolute deviations are larger. This appears to be particularly true for shoulders with atraumatic instability, where large inter-individual variation was found. Von Eisenhart-Rothe et al.8 reported the mean translation for the atraumatic instability group from 30 abduction to a position of 90 abduction and full external rotation as only 0.7mm in a posterior direction. Although the individual results were not presented, the authors stated that large anterior translations cancelled out large posterior translations, leading to a fairly centred group mean. Even in the asymptomatic shoulders in this study, the anteroposterior translation was not uniform, with 6 shoulders translating anteriorly, and 6 posteriorly. CONCLUSION Recent improvements in imaging techniques have enabled the assessment of in vivo glenohumeral translation in functional positions. This has negated some of the potential error in measuring arthrokinematics in cadaver models, where estimation of the influence of muscle activity may limit the validity of results. Open-MRI with 3-D post-processing has significant potential to clarify the mechanics of the glenohumeral joint, however, there remain a number of limitations. Clear imaging still requires multiple minutes, and as such, humeral head translations during functional movements are difficult to assess. Secondly, different research design, image acquisition, and post-processing techniques make it difficult to compare data across studies. It is apparent that arthrokinematics differ between subgroups of patients with shoulder dysfunction, and even asymptomatic individuals, and further research is required to identify the reasons for, and implications of, this phenomenon. Enough data now exists to demonstrate the limitation of the convex-concave rule as it applies to the glenohumeral joint. The information presented in this review can be used to adapt the manual therapists approach to shoulder dysfunction, highlighting the need for individualised assessment and treatment. The emphasis of rehabilitation for patients suffering from impingement and multidirectional instability should be on the active control system. As further information is gathered on arthrokinematics in vivo, conservative treatment of the shoulder should continue to evolve. REFERENCES 1. Brandt C, Sole G, Krause MW, Nel M. An evidence-based review on the validity of the Kaltenborn rule as applied to the glenohumeral joint. Man Ther. In press. 2. 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