➢ Hamstring injuries are one of the most common sports injuries and are a leading cause of time away from sports. Previous hamstring injury and age are considered the greatest risk factors for these injuries in both elite and recreational athletes.
➢ Making a diagnosis of hamstring injuries should start with an accurate history describing the mechanism of injury and a thorough physical examination. Magnetic resonance imaging and ultrasound are considered the modalities of choice for both the diagnosis and classification of these injuries. Although multiple classification systems have been described, none of them have been validated for management purposes.
➢ Surgical treatment of these injuries is governed by the anatomical location of the injury (proximal versus distal) and the nature of the injury (acute versus chronic).
➢ Operative treatment of both proximal and distal hamstring injuries is indicated when the injury involves all three muscles, when the injury involves two tendons in athletes who participate in sports requiring the maintenance of a seated position under eccentric loads to the hamstrings, and when nonoperative treatment has failed. Gluteal sciatica is another indication for operative treatment of proximal hamstring injuries.
➢ A crucial aspect of both the operative and nonoperative treatment of these injuries is the rehabilitation process. The rehabilitation program should concentrate on both eccentric strengthening and proprioceptive exercises, but there is a lack of well-designed studies to accurately assess other rehabilitation protocols.
Hamstring injuries are one of the most common sports injuries as they can affect both recreational and elite athletes1-3. These injuries have gained considerable attention in the literature because of the length of time lost from sports and the burden both on athletes and indirectly on their teams. Although fairly common, these types of injuries are considered a difficult entity to treat. Controversy exists as to the optimal treatment plan, the possible need for operative intervention, and the postoperative rehabilitation program. The first step in treating these injuries is prompt identification and accurate diagnosis. The vast majority of these injuries are muscle strains, but failure to identify the small subset of proximal and distal injuries can therefore delay the presentation to a specialist and potentially adversely affect the outcome4. These injuries have a tendency to recur, especially in elite athletes, and some patients may develop chronic symptoms.
The purposes of this review are (1) to summarize the epidemiology, classification, and evaluation of hamstring injuries; (2) to provide evidence-based principles of hamstring injury treatment, including nonoperative treatment, operative treatment, and rehabilitation protocols; and (3) to assess preventive measures and programs to decrease the incidence of these injuries in both recreational and elite athletes.
The exact rate of hamstring injuries varies as a result of different definitions of what qualifies as an injury and the diversity of populations studied5. In general, the prevalence of these injuries ranges from 8% to 25%5,6. As such, hamstring injuries represent the most common cause of time lost from sports in elite athletes6. The prevalence of these injuries is higher in sports associated with running and fast acceleration, which result in increased strain on the hamstring muscles. Hamstring injuries are frequently seen in sports such as soccer, track and field, football, gymnastics, and waterskiing1. Hamstring strains are considered the most common form of injury, whereas complete ruptures occur less frequently7.
Multiple risk factors for hamstring injuries, including both modifiable and nonmodifiable risk factors, have been described5. Modifiable factors include muscular fatigue, low muscular strength, lack of warm-up before play, higher level of competition, and low hamstring-to-quadriceps strength ratio. Nonmodifiable factors include age, previous injury, and black race8-15. Previous hamstring injury is considered the greatest risk factor and is associated with a twofold to sixfold increase in the rate of reinjury, depending on the size and severity of the initial injury16,17. Multiple studies also have shown that age is an important independent predictor of hamstring injury11,18,19. In elite athletes, especially soccer and football players, speed and kicking positions also increase the risk of injury3,8,14.
Understanding the anatomy of the hamstring complex is crucial for the treatment of these injuries. The hamstrings act as hip extenders and knee flexors because all of these muscles, except the short head of the biceps femoris, cross both the hip and knee joints (Fig. 1, Table I). The inferior gluteal and profunda femoris arteries supply these muscles, and their innervation is from the tibial and common peroneal nerves, which are branches of the sciatic nerve (Table I)20.
Mechanism of Injury
The mechanism of injury governs the location of the hamstring injury (proximal, midsubstance, or distal). Proximal hamstring injuries are usually the result of indirect trauma after a sudden, forceful maximal contraction of the hamstring muscles while the hip is in flexion and the ipsilateral knee is in extension6,21-26. Midsubstance hamstring injuries usually occur as a result of indirect trauma during sports, typically during sprinting. Distal hamstring injuries are less frequent and are associated with a mechanism similar to those seen with midsubstance hamstring injuries, which include both eccentric muscular contraction during the stance phase of running and a fall with an outstretched leg3,27.
The clinical presentation of patients with a hamstring injury is related to the grade, location, and mechanism of injury. Patients presenting with midsubstance injuries (the most common location) most commonly have a history of a sudden, acutely sharp pain in the posterior aspect of the thigh, which can be associated with the onset of ecchymosis in that area28. The majority of patients seeking medical attention because of proximal injuries are found to have moderate to severe injury patterns, whereas those with mild strains are less likely to seek medical attention because the symptoms typically are alleviated within a few days after the injury29. Patients typically describe feeling or hearing a pop in the posterior aspect of the proximal thigh and hip area, a tearing sensation associated with sudden onset of posterior thigh pain distal to the ischial tuberosity, and difficulty weight-bearing. Within a few days after the injury, patients usually notice the presence of ecchymosis over the buttocks and the posterior aspect of the thigh that can in some cases extend down into the leg. Patients often have difficulty sitting as a result of pain at the avulsion site. In the early phase of the injury, walking can be problematic for as long as four weeks and running is often impossible24,29,30. Some patients can also present with an element of neuropathic posterior thigh dysesthesia. This clinical presentation, called gluteal sciatica, is related to compression of the sciatic nerve resulting from hematoma, the retracted tendon stump, or scarring and typically involves mainly the posterior cutaneous branch of the sciatic nerve.
Patients with distal hamstring injuries can have a presentation similar to that of patients with proximal injuries. Patients with distal injuries often report weakness in knee flexion as well as feeling a pop or a snap in the posterior aspect of the thigh associated with pain and stiffness in the injured area that prevents them from weight-bearing. Depending on the degree of tear, patients may have loss of active flexion and in some cases may have difficulty with active extension. In patients with an isolated biceps femoris rupture, pain can be localized to the lateral or posterolateral aspect of the knee. In addition, cramps and spasms in the posterior aspect of the thigh are not uncommon. In patients with a complete distal hamstring tear, ligamentous instability in the ipsilateral knee must be ruled out3,27,31,32.
Patients with chronic injuries usually present because of profound weakness rather than pain. In addition, some patients present with gluteal sciatica symptoms due to excessive scarring in the area of injury and subsequent sciatic nerve tethering24,30.
Identifying a hamstring injury during physical examination can be difficult because of the deep location of the muscles within the thigh. However, there are some signs that may guide the examiner toward the diagnosis. After the injury, patients typically develop ecchymosis and swelling of the posterior aspect of the thigh, and a large hematoma might be suggestive of a more extensive injury, such as rupture of all three tendons. However, this finding is variable and is not present in all patients with hamstring injuries. In addition, patients with proximal injuries usually sit leaning toward the side in order to avoid pain at the avulsion site7,27,29,33. A palpable defect can occasionally be felt along the course of hamstring tendons, depending on the location of the injury; however, the gap could be masked by the hematoma associated with the injury29. An important part of the physical examination is testing range of motion and strength of the affected hamstring muscle and comparing these findings with those on the contralateral side. Ideally, the patient should be lying prone with the hip positioned in 0° of extension; knee flexion is then examined with resistance applied at the heel with the knee in 15° and 90° of flexion (Fig. 2). Pain provoked by the examination or weakness are considered positive findings33. Hip flexion and knee extension should be examined to test hamstring flexibility and maximum length, which can be limited by pain in patients with a hamstring injury34. A painful passive straight leg raise is expected in patients with partial hamstring injuries. Conversely, a straight leg raise with greater hip flexion on the affected side than the contralateral, uninjured side should raise the suspicion of a complete proximal avulsion.
All patients with acute hamstring injuries should be evaluated with radiographs to identify avulsion fractures of the ischial tuberosity1,35. Ischial tuberosity avulsions are more common in the pediatric population. Magnetic resonance imaging (MRI) is considered the standard modality for the diagnosis of hamstring injuries, irrespective of location (Fig. 3 and Fig. 4)7,17. This is particularly true in cases of recurrent injuries and injuries involving the deeper part of the muscle17,36. MRI is also crucial for determining the extent of the injury and for differentiating between partial and complete tears and assessing the number of tendons involved and the amount of retraction of the ruptured tendon7,36,37. MRI findings also have been shown to aid in predicting the duration of the post-injury rehabilitation period by correlation to the length and cross-sectional area of the injury15,38. Cohen et al., in a study of thirty-eight patients (forty-three hamstring injuries), found that MRI predictors for increased time away from sports included muscle retraction, >25% muscle involvement, and a long high signal on sagittal plane T2-weighted MRI39. The same authors also devised a prognostic MRI scoring system, which included the age of the patient, the number of muscles involved, the location of injury, the percentage of muscle involved, the amount of retraction, and T2 signal length (in centimeters), with scores ranging from 0 to 3 points for each parameter (Table II). This scoring system positively correlated with increased time to return to sports as patients with scores of >15 were found to have a prolonged recovery, whereas those with scores of <10 missed zero or one game only (Table II)39. Ultrasonography is another excellent tool for diagnosing hamstring injuries. Ultrasonography is easily accessible and is less expensive than MRI; however, it is highly operator-dependent, and diagnosing subtle injuries is sometimes challenging. Furthermore, in some cases, it can be difficult to differentiate scar tissue of an old hamstring injury from an acute injury36.
The spectrum of hamstring injuries comprises everything from strains to severe ruptures2. Although multiple classification systems have been described, none of them have been validated. The most commonly used system involves classification of the severity of the injury, on the basis of physical examination, as mild (grade 1), moderate (grade 2), or severe (grade 3)24. The MRI findings correlating with these definitions are a small disruption of the structural integrity of the musculotendinous unit (grade 1), a partial tear (grade 2), and a complete disruption of the unit, including an avulsion injury from the ischial tuberosity (grade 3) (Table III)19.
Another classification system is based on the anatomical location of the injury, including proximal (injury of the tendon origin at the ischial tuberosity), central (injury of the muscle belly, which is by far the most common pattern), and distal (injury of the distal musculotendinous insertion and tendons)3,23,24,32,40,41.
Wood et al. devised a classification system specifically for proximal avulsions of the hamstrings42. The system is based on the anatomical location, the degree of avulsion, the degree of muscle retraction, and the presence of sciatic nerve tethering. The system includes five types of injuries: Type 1 are osseous injuries that are more commonly seen in skeletally immature patients, Type 2 are injuries that occur at the musculotendinous junction, Type 3 are partial tendon avulsions, Type 4 are complete avulsions with no retraction, and Type 5 are complete avulsions with associated retraction. The last type is subdivided into Type 5A (without associated sciatic nerve tethering) and Type 5B (with associated sciatic nerve tethering) (Table IV).
These classification systems can be used for description purposes, but, again, none of them have been validated for guidance of either the treatment plan or the postoperative rehabilitation protocol for injured patients.
Although much of the attention in the literature has been directed toward the operative treatment of hamstring tears, there is a general consensus that nonoperative treatment is indicated for partial injuries, midsubstance injuries, complete single-tendon proximal injuries, and two-tendon proximal injuries with <2 cm of retraction7,30,40,42,43. However, there is no consensus on the nature of this conservative treatment.
The goal of nonoperative treatment is to control the pain and inflammation caused by the acute injury. Nonsteroidal anti-inflammatory drugs have been implemented as an option for the inhibition of inflammatory cells and pain perception, but the effect of such drugs on reducing the symptomatic aspects of hamstring injuries has not been proven44. Intramuscular corticosteroid injections have been found to accelerate the return to the preinjury level of competition, but studies have shown both favorable and unfavorable effects on the healing process of the injured muscle, and this method remains controversial45,46. Recent attention has been directed toward the use of platelet-rich plasma injections for the treatment of hamstring injuries, with promising results in some studies, but more trials are required before this modality can be globally recommended47,48. Recommendations also point to the avoidance of unnecessary immobilization of the injured limb to avoid atrophy of the muscle49.
Treatment modalities such as ultrasound stimulation, cold and heat compressors, and massage have been described for the treatment of hamstring injuries, but no validated results have been described50. A general recommendation for the treatment of muscle injuries, although not fully proven for hamstring injuries, is limiting the extent of injury according to the RICE (rest, ice, compression, and elevation) principle49. Another important aspect of the nonoperative treatment of these injuries is an adequate rehabilitation process with programs concentrated on strengthening and regaining range of motion of the extremity to prevent unwanted sequelae and improve the general outcome4,7,33,43,51-54.
There is no clear consensus regarding the operative treatment of hamstring tears5. Many techniques have been described; however, to our knowledge, no randomized controlled trials have been performed in an attempt to standardize the operative and rehabilitation protocols used by orthopaedic surgeons. In this section, we will divide the operative treatment of proximal and distal avulsions into acute (up to four weeks) and chronic (greater than four weeks) (Fig. 5).
Acute Proximal Hamstring Injuries
Operative treatment is recommended for acute proximal hamstring injuries in which all three hamstring tendons are avulsed from the ischial tuberosity. Operative treatment also can be considered in cases of high-grade two-tendon injuries with >2 cm of retraction in athletes participating in sports requiring the maintenance of a seated position under eccentric loads to the hamstrings (e.g., waterskiing, rodeo, kite surfing, etc.)7,30,40,42,43.
The goal of treatment of these injuries is to restore muscular and osseous attachments. Several techniques have been described, and most involve the use of suture anchors for approximation of the muscular rupture or avulsion to the ischial tuberosity4,23,30,55-57. However, the use of heavy sutures that are passed through drill-holes in the ischial tuberosity also has been described21. Large osseous avulsions of the proximal origin of the hamstrings are treated with open reduction and internal fixation (e.g., screw fixation) of the osseous fragment.
The surgical repair of acute complete proximal hamstring injuries with use of suture anchors has been associated with good to excellent results as reported in the literature, with patients returning to the preinjury level of activity at a mean of five months after surgery4,23,30,40,42,55-60 and with postoperative strength returning to 78% to 100% of that on the contralateral side as demonstrated with isokinetic testing42,55,58-60. In comparison, after nonoperative treatment, hamstring strength on the involved side has been found to be as low as 30% when compared with that on the contralateral side61. Primary repair (including end-to-end repair or repair with use of bone drill-holes) without suture anchors has been shown to be less successful; in one study, only five (63%) of eight patients had comparable strength to the contralateral, uninjured side and the other three patients (38%) continued to have substantial post-repair weakness21.
Lempainen et al. reported the results of revision repair in a series of four patients who had a rerupture of the proximal origin of the hamstring muscles after failed primary repair62. An iliotibial band autograft was used to augment the repair. Of the four recreational athletes in that series, three were able to return to their sporting activities (golf, running, and downhill skiing) at a mean of twelve months (range, six to eighteen months) after the operation. None of the patients had been able to participate in their recreational sports before the reoperation.
Chronic Proximal Hamstring Injuries
Proximal hamstring injuries can be misdiagnosed or even missed altogether, leading to a delay in operative intervention63. Operative intervention is usually considered for chronic injuries if the patient cannot return to his or her desired activities because of persistent weakness, pain, or both, despite appropriate nonoperative treatment40,55,64. In addition, the presence of gluteal sciatica is also an indication for operative treatment.
On the basis of the available literature, it is believed that acute injuries are easier to repair, and early repair has shown excellent functional results65. In contrast, delayed operative repair of complete proximal ruptures has yielded worse postoperative strength, with the potential for sciatic nerve involvement by surrounding scar formation30,42,66. Aldridge et al.64 and Sallay et al.55 recommended neurolysis of the sciatic nerve for all chronic cases. In contrast, Lempainen et al. recommended selective neurolysis only in cases in which scar tissue was tethering the sciatic nerve40. The main challenge in these chronic cases is that the anatomical reattachment of torn tendons is much more difficult than in acute cases as all of the tendons are scarred together and can be difficult to isolate. Often, the knee has to be maintained flexed and immobilized to achieve apposition and to reduce the tension on the repaired tendons until complete healing65,67. Most authors have preferred to use suture anchors in cases in which avulsed tendons can be approximated to the ischial tuberosity23,30,40,42,43,55,64,67. Various surgical techniques and reconstructions have been described to deal with the injury in cases in which it is not possible to reduce the torn tendons back to the ischial tuberosity. Sallay et al. described a technique of distal hamstring fractional lengthening to aid in reapproximation of the proximal tendons to the ischial tuberosity, with good results55,66. Folsom and Larson used Achilles tendon allograft to augment repair in one patient and to bridge the defect in three other patients, with three of the four patients returning to sports4. Marx et al. also used an Achilles tendon allograft for proximal hamstring tendon reconstruction in a case series of two patients68. Although the treated side was weaker than the contralateral side postoperatively, both patients returned to the preinjury level of activity68. Lempainen et al. described the use of fascia lata autograft in one patient, who had a satisfactory result and returned to sports62. Sarimo et al. also described the use of fascia lata autograft to bridge the gap30. Orava and Kujala used fascia lata and plantaris autografts to strengthen the fixation in two patients in their series, with moderate to good outcomes21.
Acute Distal Hamstring Injuries
Distal hamstring injuries are extremely rare, and only a few reports have been published in the literature3. These injuries most commonly involve the biceps femoris tendon and, to a lesser extent, the semitendinosus and semimembranosus tendons. In the largest case series of which we are aware, Lempainen et al. reported on eighteen patients who had operative treatment of distal partial and complete hamstring tears3. Only four of these injuries were acute: two were avulsions involving the biceps femoris and semitendinosus tendons, and the other two were complete ruptures at the musculotendinous junction of the biceps femoris muscle (one patient) or the semitendinosus (one patient). In the two patients in whom the tear was located at the musculotendinous junction, operative repair was achieved with use of sutures after excision of scar tissue. In the patient with avulsion of the biceps tendon, reinsertion of the tendon to the head of the fibula was done with a suture anchor. However, anatomical reinsertion without considerable tension was not possible in the patient who had avulsion of the semitendinosus tendon. In that case, the torn semitendinosus tendon was reinserted with sutures to the tendinous part of the sartorius muscle. All four patients had excellent outcomes after treatment (i.e., all patients were asymptomatic and returned to the preinjury level of sports). Kusma et al. used suture anchors to treat acute distal biceps femoris avulsion in one patient, who was able to return to competitive sports69. Other authors have advocated for primary repair with use of heavy sutures and have reported good outcomes27,31,70-72.
Chronic Distal Hamstring Injuries
There is a paucity of literature on this entity. Lempainen et al. reported on fourteen patients who underwent operative treatment of chronic distal hamstring tears3. Nine of these injuries involved the biceps femoris muscle, and five involved the musculotendinous junction of the semimembranosus. Primary repair after excision of scar tissue was performed for the treatment of all biceps femoris injuries and three of the semimembranosus injuries. The remaining two partial semimembranosus injuries were associated with muscle cramps and spasms, and in those cases only a tenotomy of the distal semimembranosus tendon was performed, without any repair. Twelve of fourteen patients had good to excellent results; however, the two patients in whom semimembranosus injuries were treated with tenotomy had poor results. In that series, only four patients did not manage to return to their previous level of sports activity; all of these failures involved semimembranosus injuries.
Proximal Hamstring Syndrome
The presence of persistent pain at the ischial tuberosity and proximal hamstring regions may be associated with prior hamstring injury or tendinopathy and has sometimes been termed the hamstring syndrome73-75. Different authors have postulated that this entity occurs secondary to tendinous band entrapping or scarring around the sciatic nerve due to fibrosis of the injured hamstring tendons73,74,76. This scarring leads to tethering of the nerve, especially with specific positions, including sitting or positions involving stretching of the hamstring muscles, and thus the associated sciatica pain73,74. Lempainen et al. evaluated the histopathological changes of these tendons75. All surgical samples showed signs of tendinosis. In that series, eighty (89%) of ninety patients had good to excellent results (with the patients being asymptomatic and returning to the preinjury level of sports) following semimembranosus tenodesis by reattachment to the biceps tendon without neurolysis. Other authors have reported good results in association with either tendon debridement74 or tenodesis73. In contrast to Lempainen et al., those other authors concluded that a complete neurolysis of the sciatic nerve from adherent tissue was an essential component of the operative treatment.
The main purpose of any rehabilitation program following the operative or nonoperative treatment of a hamstring injury is to restore the preinjury level of function. Multiple rehabilitation and postoperative management protocols have been described in the literature4,7,33,43,51-54. No specific protocol has been validated as being superior to the others on the basis of a randomized controlled trial. Regardless of the specific rehabilitation protocol, neuromuscular control and eccentric strengthening exercises, such as Nordic hamstring exercises, are repeatedly recommended to be part of rehabilitation programs for patients with acute hamstring injuries6,77-79. The theory is that stretching and strengthening in the rehabilitation process of acute hamstring injuries can help to remodel and align collagen fibers in the scar tissue80. Other authors have theorized that lumbopelvic neuromuscular control, including anterior and posterior pelvic tilt, is key for optimizing hamstring muscle function and therefore have suggested a progressive agility and trunk stabilization (PATS) rehabilitation program81,82. Sherry and Best showed that individuals with an acute hamstring injury who were managed with the PATS program had a lower reinjury rate than patients who were managed with a progressive stretching and strengthening program52. In contrast, Silder et al. showed no significant difference in the reinjury rate between individuals with an acute hamstring injury who completed the PATS program and individuals who completed a progressive running and eccentric strengthening program53. Regardless of the rehabilitation protocol used, patients should undergo an assessment before returning to sports, but there is no good evidence in the literature for a functional test that can predict or guide the timing of returning to play with a low reinjury rate. Some general criteria that have been mentioned in the literature for the return to sports include full strength and range of motion without pain, a symmetrical knee flexion angle of peak torque, and comfortable replication of sport-specific movements at the competition level54.
Although hamstring injuries are a common entity, the literature lacks evidence on the effectiveness of prevention programs, in part because of the paucity of well-designed studies on this topic83. It has been shown that the rate of reinjury is about 12% to 41%, with the second injury being more severe and doubling the time lost from sports when compared with the initial injury13,14,33. Multiple studies have shown the effectiveness of eccentric strengthening in reducing the risk of sustaining this type of injury, both as a preventive measure and in patients with established hamstring strains, especially when preceded by an adequate duration of warm-up stretching6,84,85. However, Goldman and Jones, in a systematic review, highlighted that there is insufficient evidence to support the preventive effect of eccentric training programs83. Exercises directed at trunk stabilization and core strengthening also have been shown to reduce the risk of injuries in this muscle group50,52. The use of neoprene compressive-type garments, incorporation of warm-up and cool-down exercises before and after play, and joint proprioception training have been reported as potential factors that can decrease the risk of hamstring injuries and their recurrence, but few studies have been performed to support these statements5,77,86,87. More well-designed, randomized clinical trials are required before specific preventive recommendations can be implemented widely.
A great deal of controversy exists in the literature concerning the treatment and rehabilitation protocols for hamstring injuries, but general principles can guide the orthopaedic surgeon during the evaluation process. Complete injury of all three hamstrings, proximal grade-2 tendon injuries with >2 cm of retraction in athletes who participate in sports requiring the maintenance of a seated position under eccentric loads to the hamstrings, and gluteal sciatica in patients with distal injuries are considered to be indications for the operative treatment of hamstring injuries (Fig. 5). The rehabilitation period should concentrate on eccentric and proprioceptive exercises. Preventive measures have been recommended, but none of these preventive programs have been proven in the literature. The lack of Level-I and II studies in the current literature limits the strength of recommendations for both treatment and rehabilitation protocols and points out the need for further well-designed prospective studies and randomized clinical trials to optimize the treatment of hamstring injuries.
Source of Funding: No external funds were received in support of this study.
Investigation performed at the Division of Orthopaedic Surgery, Department of Surgery, McGill University Health Center, Montreal, Quebec, Canada
Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. None of the authors, or their institution(s), have had any financial relationship, in the thirty-six months prior to submission of this work, with any entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. Also, no author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.
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