1.1

Introduction

In 1890, Broca and Hartman hypothesized that recurrence after an initial traumatic anterior-inferior dislocation of the shoulder was related to the presence of damage to the glenoid labrum and a posterior defect of the head of the humerus. In 1950, De Palma criticized this hypothesis and suggested that recurrent glenohumeral instability was predominantly due to a neuromuscular imbalance caused by rotator muscle damage at the time of the initial trauma. Isokinetic evaluation is now acknowledged to be a reliable technique for evaluating the performance of the shoulder rotator muscles. It enables the detection of impairments in some muscle groups and the identification of disease-induced perturbations of the agonist-antagonist balance.

The objective of the present study was to:

• •
  

  study the possible impact of post-traumatic instability on the muscle balance between the internal rotators (IRs) and external rotators (ERs) of the injured shoulder as a function of dominance, the number of dislocations and the severity of any associated bone damage;

  
  
• •
  

  and specify the indication for isokinetic evaluation.

1.2

Materials and methods

We performed a prospective study between April 2004 and January 2009.

1.2.1

Population

We performed a comprehensive, isokinetic assessment of both shoulders on 102 male patients, all of whom presented secondary instability following one or more anterior or anterior-inferior dislocations of traumatic origin. The mean patient age was 24.8 (range: 16–47) and the morphological characteristics of the study population are summarized in Table 1 . The causes of instability are detailed in Fig. 1 .

Table 1

Morphological characteristics of the study population.

Patients	Age (years)	Weight (Kg)	IMC	Number of recurrences
Dominant side injured ( n = 64)	23	72.3	23.3	4 (2–10)
Non dominant side injured ( n = 38)	26	73.5	23.7	4 (2–8)

 

The causes of instability.

1.3

Results

1.4

Discussion

The objective of the present work was to study the possible impact of recurrences of post-traumatic anterior and anterior-inferior dislocations on the shoulder IR/ER muscle balance.

Our study concerned only male patients; this type of recruitment was prompted by literature data showing that post-traumatic dislocations are more frequent in men. Zachilli and Owens found that males accounted for 71.8% of the patients attending the emergency room for this condition (with an annual incidence of 23.9 per 100,000 subjects).

The mean patient age in our study was 24.8 (range: 14–47), which is similar to that generally found by other authors. Zachilli and Owens observed that 46.8% of trauma incidents concerned the 15–29-age class. Owens et al. reported an annual incidence of 169 per 100,000 subjects for US military personnel and 435 per 100,000 subjects for military cadets.

Sport was the most frequent cause of dislocation and accounted for 78% of the cases in our study ( Fig. 1 ); this value was 48% for Zacchilli and Owens , 47% for Simonet et al. and 96% for Owens et al. . These differences can be explained by the nature of the patient recruitment: the values reported by Zacchilli and Owens and Simonet et al. corresponded to emergency room admissions, whereas Owens’ et al. study was performed in military academies. In our present study, many of the subjects were students and played sport (kayaking, judo, handball, soccer, etc.) on a regular basis; indeed, a good proportion were high-level sportsmen in regional and national squads.

The management of shoulder instability has been addressed by many authors . Hovelius et al. estimated that half of the subjects aged 12 to 25 undergoing rehabilitation after a first episode of instability did not suffer from recurrence and that compliance with post-traumatic immobilization did not appear to influence the risk of recurrence. In contrast, Itoi et al. recommended immobilization in an external rotation brace, which appeared to be associated with a decreased risk of recurrence. However, Finestone et al. did not find a significant relationship between internal or external immobilisation and the rate of recurrence after initial dislocation. In a study with 7 years of follow-up of anterior glenohumeral dislocations, Tsai et al. noted that 65% of the subjects reported instability during physical activity.

In fact, it appears that age, side (dominant vs. non-dominant), gender, the severity of initial bone damage (a posterior chip fracture or damage to the lower edge of the glenoid cavity) and the level and type of sport are more important predictive factors than immobilization in the risk of recurrence . These various criteria were met in our present study: the population was young (mean age: 24.8), with a high proportion of keen sportsmen; the trauma was on the dominant side in 63% of cases and was associated with a posterior chip fracture of the humeral head in 33% of cases, damage to the lower edge of the glenoid cavity in 20% of cases and both types of damage in under 10% of cases.

Although isokinetic testing of the shoulder has been extensively studied , there are still a number of disparities in terms of the body positions, velocities and protocols used. However, there appears to be a consensus in favour of the Davis modified position as the reference position for the evaluation of painful and/or unstable shoulders. The supine position with the arm abducted at 90° is used above all when evaluating throwers and in the absence of an underlying pathology.

Concentric mode measurements are reproducible, as long as the protocol is rigorously adhered to . In our study, we used the modified Davis position. We always tested the injured shoulder first, while limiting the movement to pain-free amplitudes and allowing the subject to warm up for as long as required (in order to dispel any apprehension). We performed all our measurements in a concentric mode, in order to maximize reproducibility and decrease the risk of injury.

Many studies have tried to establish reference values for isokinetic measurements of rotator muscles in the healthy shoulder. Different parameters influence the outcome of these measurements, such as age, gender, the subject’s morphology, dominance vs. non-dominance, the type of sporting activity, the subject’s occupation, the dynamometer used for the tests, the angular velocities and the subject’s position during the test.

In sedentary subjects, one does not observe a significant difference between the dominant and non-dominant arms. In contrast, higher IR torque values are found for the dominant arm in sportspeople practicing asymmetric sports, such as tennis , baseball and volleyball . Consequently, ER/IR ratios are lower. Some authors consider that imbalance may cause the shoulder pain frequently experienced in these groups of sportspeople .

Research on imbalance of the shoulder IRs and ERs in the aftermath of dislocation has yielded contradictory results; some authors consider that instability is favoured by an impairment of the IRs, whereas others believe that impairment of the ERs is to blame . These studies concerned small, heterogeneous populations (a mixture of males and females, sportspeople and sedentary subjects, etc.) in whom glenohumeral instability was not always due to trauma. Our study solely concerned men presenting with post-traumatic anterior or anterior-inferior instabilities. In this well-defined setting, we did not find any significant difference in the ER/IR ratios (measured in concentric mode) when comparing the healthy side and the injured side, regardless of whether the injured shoulder was dominant or not, the number of dislocations and the severity of bone damage observed.

We consider that isokinetic testing is of value in cases in which the traumatic origin of the instability is uncertain and radiographic screening is negative. In these cases, the discovery of muscle imbalance would guide the physician’s choice of a rehabilitation programme. In contrast, a confirmed lack of muscle imbalance would provide an additional argument in favour of joint injury and would prompt the physician to perform additional radiological screening for labral damage.

1.5

Conclusion

In the present study population, post-traumatic dislocations had no impact on the muscle balance of the shoulder rotators (as measured in concentric isokinetic mode).

These observations validate Broca and Hartman’s hypothesis in a post-traumatic setting, whereas the theory of “muscle imbalance” developed by De Palma appears to apply only to non-traumatic instabilities.

These observations prompt us to question the usefulness of rehabilitation (and notably specific strengthening of the IRs) after an episode of post-traumatic instability, as is generally recommended. Indeed, our findings raise the question of whether proprioceptive work alone might be sufficient.

In any case, we consider that concentric-mode, isokinetic testing is not essential in patients with post-traumatic anterior or anterior-inferior dislocations.

Disclosure of interest

The authors declare that they have no conflicts of interest concerning this article.