Abstract
Aims
This prospective cohort study aims to determine if the size of the tendon gap following acute rupture of the Achilles tendon shows an association with the functional outcome following non-operative treatment.
Patients and Methods
All patients presenting within two weeks of an acute unilateral rupture of the Achilles tendon between July 2012 and July 2015 were considered for the study. In total, 38 patients (nine female, 29 male, mean age 52 years; 29 to 78) completed the study. Dynamic ultrasound examination was performed to confirm the diagnosis and measure the gap between ruptured tendon ends. Outcome was assessed using dynamometric testing of plantarflexion and the Achilles tendon Total Rupture score (ATRS) six months after the completion of a rehabilitation programme.
Results
Patients with a gap ≥ 10 mm with the ankle in the neutral position had significantly greater peak torque deficit than those with gaps < 10 mm (mean 23.3%; 7% to 52% vs 14.3%; 0% to 47%, p = 0.023). However, there was no difference in ATRS between the two groups (mean score 87.2; 74 to 100 vs 87.4; 68 to 97, p = 0.467). There was no significant correlation between gap size and torque deficit (τ = 0.103), suggesting a non-linear relationship. There was also no significant correlation between ATRS and peak torque deficit (τ = -0.305).
Conclusion
This is the first study to identify an association between tendon gap and functional outcome in acute rupture of the Achilles tendon. We have identified 10 mm as a gap size at which deficits in plantarflexion strength become significantly greater, however, the precise relationship between gap size and plantarflexion strength remains unclear. Large, multicentre studies will be needed to clarify this relationship and identify population subgroups in whom deficits in peak torque are reflected in patient-reported outcome measures.
Cite this article: Bone Joint J 2017;99-B:87–93.
The Achilles tendon is the strongest and most frequently ruptured tendon in the body, with an annual incidence of 21 ruptures per 100 000 person-years.1 This incidence is increasing year on year.1 Rupture of the Achilles tendon typically affects people in the fourth and fifth decades of life who participate in intermittent athletic activity, with men more frequently affected than women.2-4 Though this is a common injury, there is debate regarding the optimum management. In the past decade, the body of research supporting the use of non-operative management has grown, with large studies suggesting no significant difference in rate of re-rupture following operative and non-operative management.5-7 This knowledge, combined with the relatively high rates of complication following surgery, has led to a sharp increase in the proportion of patients being treated non-operatively.8-10
Much of the current literature focuses on the complications following treatment, such as tendon re-rupture and post-operative infection, with very little documentation of the functional outcomes in cases of successful treatment.11-13 Detailed knowledge of these outcomes and any factors which influence them is key in managing patient expectations, engaging in shared decision making and establishing informed consent. In recent years, some centres have begun using ultrasound measurement of the size of the tendon gap to assist in making treatment decisions. In the United Kingdom promising outcomes have been reported when a tendon gap of 10 mm is used as a threshold value beyond which operative management is chosen.14
This prospective cohort study aimed to investigate the relationship between the size of the tendon gap in the ruptured Achilles tendon and long-term functional outcome in patients treated non-operatively. Outcome was assessed by dynamometric testing of the peak torque of plantarflexion and the Achilles tendon Total Rupture score (ATRS)15 at six months following the completion of rehabilitation. These measures are validated for determining clinical outcome.15-21 We sought to determine if tendon gap size correlates with peak torque deficit on the injured side compared with the uninjured side at six months; if tendon gap size inversely correlates with the ATRS at six months; if patients with tendon gap size of 10 mm or more experienced greater reduction in plantarflexion strength and ATRS at six months compared with those with tendon gaps < 10 mm.
Patients and Methods
This prospective cohort study is reported in accordance with the Strengthening the Reporting of OBservational studies in Epidemiology (STROBE) statement for cohort studies.22 We included adults with primary unilateral rupture of the Achilles tendon presenting through the foot and ankle service at our institution between July 2012 and July 2015. Before the injury the patients were fully mobile, independent of walking aids and had no significant pre-existing disease or injury affecting either lower limb. Patients who elected surgical repair or presented late (after 14 days) were excluded. The study was reviewed and approved by the regional research ethics committee (ref:12/EE0167).
Patients were clinically assessed by a consultant orthopaedic surgeon specialising in foot and ankle surgery (AHNR). They then underwent ultrasonographic evaluation of the affected tendon by a consultant musculoskeletal radiologist specialising in ultrasound (LB). Dynamic imaging was used to confirm the diagnosis of tendon rupture. The gap between the two tendon ends was measured in both neutral and fully plantarflexed positions of the ankle, with the knee in both full extension and 90° of flexion. Scans were performed on a Toshiba Aplio 500 apparatus using a combination of 10MHz and 18MHz linear array transducers (Toshiba Medical Systems, Crawley, United Kingdom).
Patients underwent dynamometric testing of plantar flexion strength in the uninjured ankle during the initial clinic visit. This was performed using a computerised isokinetic Cybex dynamometer (Cybex International, Inc., Medway, Massachusetts) in the prone position with a single speed protocol at 60°/second. Two trials consisting of ten repetitions were performed followed by a final trial of five repetitions in order to determine accurately the mean peak torque generated by the musculo-tendinous unit. Treatment involved sequential casting with the injured side placed in a below knee full equinus cast for four weeks. The foot was then placed in semi-equinus and neutral casts for two weeks each. This was followed by physiotherapy with a rehabilitation programme specific to Achilles tendon ruptures, with initial focus on range of movement and proprioception exercises, progression to resistance band exercises and finally eccentric contraction exercises. Patients were followed up six months following completion of the rehabilitation programme with dynamometric testing of the injured side. Peak torque values obtained from the injured side were then compared with the contralateral side at the time of injury, with any difference between the two values expressed as a percentage in order to account for variability in the raw values of torque between individuals. The ATRS questionnaire was also completed at this follow-up. The ATRS was undertaken by a clinical research nurse (JE) and dynamometry by a clinician (JEL). Both were blinded to the ultrasound measurements. The same clinician undertook each process throughout the study in order to eliminate interobserver variability.
A total of 63 patients were examined for eligibility, of whom 18 were excluded; 11 were found to have partial ruptures on dynamic ultrasound assessment, five opted for surgical treatment, one had bilateral injuries and one a fused ankle joint. In all, seven patients were lost to follow-up, leaving 38 patients who completed the study. There were 29 men and nine women, with a mean age of 52 years (29 to 78) (Fig. 1).
Fig. 1
Diagram showing flow of patients through the study. ATRS, Achilles tendon total rupture score.
Statistical analysis
Working on the basis of a 5% significance level (α = 0.05) for reporting correlation in a two-sided test, a sample size of 38 patients was calculated as sufficient to detect a correlation coefficient of magnitude 0.5, or greater, between tendon gap size and peak torque deficit with 90% power (β = 0.1). A Kendall’s tau correlation coefficient was used to test against our null hypotheses that there is no correlation between tendon gap measured on ultrasound with deficit in plantar flexion strength at six months or ATRS at six months. The Shapiro-Wilk test of normality and the F-test of equality of variance were used before performing the Student’s t-test to test for significant difference in peak torque deficit or ATRS between patients with large (≥ 10 mm) and small (< 10 mm) ruptures. Paired t-tests were used to test the effect of knee and ankle position on the distance between ruptured tendon ends. A p-value < 0.05 was deemed to be significant. Statistical analysis was performed using version 3.2 of R Project for Statistical Computing (R Foundation, Vienna, Austria).
Results
There were no exclusions from analysis of the 38 patients completing the protocol.
Ultrasound examination
A total of 18 patients sustained ruptures on the dominant side, and 20 on the non-dominant side. With the knee in extension, the mean distance between ruptured tendon ends with the ankle at neutral and in full plantar flexion was 8.9 mm (0 to 34.3) and 6.2 mm (0 to 32.6), respectively. With the knee in 90° flexion, the mean distance between ruptured tendon ends in neutral and full plantar flexion was 7.1 mm (0 to 36.4) and 5.1 mm (0 to 31.6), respectively. The decrease in distance achieved by plantar flexion in both positions was significant (p < 0.001, paired t-test) (Fig. 2). Placing the knee in flexion significantly reduced the distance between ruptured tendon ends with the ankle in both the plantarflexed (p = 0.043, paired t-test) and neutral positions (p = 0.007, paired t-test) (Fig. 3).
Fig. 2
Plot showing distance between tendon ends with the ankle in both neutral and plantar flexion (PF), with the knee in extension for each patient.
Fig. 3
Plot showing the distance between tendon ends in knee extension and flexion, with the ankle in neutral for each patient.
Peak torque deficit
There was no significant correlation between peak torque deficit and tendon gap at injury measured in any position (Table I). Patients with a large tendon gap at injury (≥ 10 mm with the ankle in the neutral position and the knee in extension) had a mean peak torque deficit of 23.3% (n = 15; 7% to 52%). In contrast, the mean peak torque deficit of those with small tendon gaps (< 10 mm) was 14.3% (n = 23; 0% to 47%), representing a statistically significant difference between the two groups (p = 0.023, Student’s t-test) (Fig. 4).
Fig. 4
Graph showing the effect of gap size with the ankle in neutral and the knee extended on peak torque deficit, with standard error bars. *Denotes significance at the 95% confidence interval (p = 0.023, Student’s t-test.
Table I
Kendall’s Tau correlation coefficient for gap size versus outcome measures
| τ vs peak torque deficit | τ vs ATRS | ||
|---|---|---|---|
| Lower limb position for gap measurement | |||
| Knee extended ankle neutral | 0.103 | 0.016 | |
| Knee extended ankle plantarflexed | 0.023 | 0.018 | |
| Knee flexed ankle neutral | 0.298 | -0.134 | |
| Knee flexed ankle plantarflexed | 0.223 | -0.133 | |
-
ATRS, Achilles tendon total rupture score
ATRS
There was no significant correlation between ATRS and tendon gap size measured in any position (Table I). Those with a large tendon gap at injury (≥ 10 mm with the ankle in the neutral position and the knee in extension) had a mean score of 87.2 (74 to 100) and those with a small gap (< 10 mm) a mean score of 87.4 (68 to 97), representing no statistical difference between the two groups (p = 0.467, Student’s t-test) (Fig. 5).
Fig. 5
Graph showing the effect of gap size on Achilles tendon total rupture score (ATRS) with standard error bars.
There was no demonstrable correlation between ATRS and peak torque deficit (τ = -0.305). There was no significant difference in outcome between ruptures affecting the dominant (n = 20) and non-dominant (n = 18) sides (p = 0.459, Student’s t-test). Age had no correlation with either peak torque deficit (τ < 0.001) or ATRS (τ = 0.082). There were no re-ruptures in the study group.
Discussion
This study aimed to investigate the relationship between the gap measured by ultrasound in a ruptured Achilles tendon and the functional outcome six months after the completion of non-operative treatment. We found no correlation between gap size and deficit in plantarflexion and there was no correlation between gap size and ATRS. Our results identified that a gap size ≥ 10 mm with the knee extended and the ankle in the neutral position is a threshold, beyond which deficits in plantarflexion strength following treatment become significantly greater.
We believe that this finding of a threshold value for tendon gap size is in keeping with what is known about the physiological and biomechanical consequences of tendon rupture. Human skeletal muscle follows a non-linear force-length relationship, with a gradual increase in tension (force) during sarcomere lengthening, followed by a plateau, and finally a rapid decline as the length exceeds that which is biomechanically optimal.23 Different muscle groups operate across different parts of this force-length curve, with gastrocnemius and soleus operating primarily on the ascending and plateau sections in vivo.24,25 Several studies have shown lengthening of the Achilles tendon following rupture, with subsequent impairment of function due to a right shift in muscle fibre position on the force-length curve.26-31 As tendon healing, in simple terms, relies on progressive remodelling of the haematoma formed between tendon ends, it stands to reason that the size of the tendon gap will determine the extent of tendon lengthening.32 As tendon gap size is gradually increased, the muscle’s position on the force-length curve will shift to the right so that muscle strength is initially unaffected, with the muscle continuing to operate on the ascending and plateau sections, then becomes suddenly reduced as the lengthening causes the muscle to operate on the descending part of the curve. This model is compatible with our study findings of a sudden drop off of strength, at a gap ≥ 10 mm with the ankle in the neutral position, rather than a direct linear correlation between gap size and torque deficit.
The clinical significance of this biomechanical alteration remains unclear. Although the difference in plantarflexion strength between patients with small (< 10 mm) and large (≥ 10 mm) tendon gaps was statistically significant, the similarity in ATRS scores between the two groups casts doubt on the value of gap size as a prognostic indicator. Both ATRS and plantarflexion strength have been validated as clinical outcome measures.15-21 We are not the first to demonstrate a lack of correlation between these two measures. In 2013, a study by Rosso et al33 comparing functional outcomes in three treatment groups of patients with acute rupture of the Achilles tendon found no correlation between clinical scores, including ATRS, and biomechanical outcomes. It remains unclear if this lack of correlation would continue to hold true for specific subpopulations of patients such as athletes, or those undertaking heavy manual labour. One would expect ATRS to be more affected by a decrease in the peak torque generated by ankle plantarflexion in more active subpopulations than in the typical patient. Large multicentre studies assessing the functional outcomes of patients with acute rupture of the Achilles tendon will identify any such subpopulations and elucidate the level of peak torque reduction affecting an individual patient’s normal activity level. No such studies have, as yet, been performed.
Our study used dynamic ultrasound scanning for tendon imaging, which has been shown to be sensitive and specific for detecting full- and partial-thickness ruptures of the Achilles tendon, as well as reliable for measuring the distance between ruptured tendon ends.34-38 Prior to this study there had been varying reports on how knee flexion affects the distance between the two ruptured tendon ends, with two studies of living subjects drawing contradictory conclusions.39,40 Quereshi et al39 had previously described a study of 26 patients where a significant 3 mm decrease in tendon gap was achieved when changing the position of the knee from full extension to 90 flexion, with the ankle in maximal plantarflexion. Conversely, Trickett et al40 studied 35 patients and achieved a statistically insignificant 0.51 mm mean reduction in gap size when the knee was placed from full extension into 90° flexion with the ankle in plantarflexion.
To the best of our knowledge, ours is the largest non-cadaveric study to date to show a significant reduction in tendon gap size with changes in ankle and knee position. The tendon gap is smallest (mean 5.1 mm) with the knee in 90° of flexion and the ankle in full plantar flexion. This compares with a mean gap of 8.9 mm with the knee extended and the ankle in neutral. Interestingly, changing the position of the knee from full extension to 90° flexion, with the ankle in maximal plantarflexion, only reduced the tendon gap by a mean of 1.1 mm (6.2 mm vs 5.1 mm). While statistically significant (p = 0.043), this is a small difference, and the benefits of this decrease should be carefully weighed against the increased risks associated with above knee casting, which is used by some surgeons in the management of ruptures of the Achilles tendon.41,42
The main limitation of our study is that we did not use a treatment programme that involved early mobilisation, and instead treated patients with eight weeks’ sequential casting, followed by a rehabilitation programme specific to ruptures of the Achilles tendon. There has been a lot of recent literature demonstrating the benefit of accelerated functional rehabilitation following a rupture of the Achilles tendon, resulting in a recovery that is both superior and achieved in a shorter time than with conventional rehabilitation.6,26,43,44 Whilst our unit has now adopted functional rehabilitation, this was not the case at the time of study design in 2012, when the evidence supporting functional rehabilitation was not as substantial.6,26,43,44
A second potential limitation was the use of a single session of isokinetic dynamometry at six months following treatment. This interval was chosen following a study validating the use of dynamometry at six months as a clinical outcome measure.19 It is noteworthy, however, that a study by Lantto et al45 in 2015 showed a slight increase in strength between dynamometry at six months and repeated measurements eight months and ten years later.
Although the ATRS is a simple, extensively validated questionnaire and was administered by the same clinician throughout the study, it is still subject to some limitations. In its development and validation, the authors suggest a ten-point difference to be clinically relevant.15 As a result, a precise correlation between ATRS and plantarflexion strength deficit is unlikely as a result of the natural variation in scores one expects within a patient population.
In conclusion, we feel that this study provides evidence that tendon gap size at the time of injury shows an association with plantarflexion strength following non-operative treatment. However, the evidence is lacking for other measures of clinical outcome, such as the ATRS. We believe that measuring tendon gap with the knee in extension and ankle in neutral can play an important role when making treatment decisions in the more active subpopulations of patients who are more likely to suffer as a result of decreased strength. However, larger studies of functional outcomes in these patients will be needed to assess this fully.
Take home message: - Patients with a tendon gap of 10 mm or more experience significantly reduced plantarflexion strength when compared with patients with a smaller tendon gap. - This is not reflected in patient-reported outcome, which does not change with tendon gap size.
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Author contributions:
J. E. Lawrence: Recruited patients for the study, Performed the literature search, Gathered dynamometry data, Wrote the manuscript.
P. Nasr: Designed the study, Gained ethical approval, Edited the manuscript.
D. M. Fountain: Performed the statistical analysis.
L. Berman: Advised on study design, Performed the ultrasound examinations for the study.
A. H. N. Robinson: Supervised the study and edited the manuscript.
The authors wish to thank J. Ellis, BSc, Research Nurse, Addenbrooke’s Hospital, Cambridge, for her help with gathering patient-reported outcome data.
No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.
This article was primary edited by G. Scott.
