ACL injury
July 8, 2019
An ACL injury is very common in sports and recreational activities due to the high stresses combined with the great mobility of the knee.
This frequency makes the topic widely discussed and researched in science.
In the latest survey "Sport Switzerland" by the Observatory for Sport and Physical Activity (Federal Office for Sport FOSPO) from 2014, with the aim of providing a detailed picture of the current sports activities of the Swiss population, 35.4% of respondents said they ski [1]. Alongside hiking (44.3%), cycling (38.3%), and swimming (35.8%), skiing is one of the most popular sports among the Swiss [1].
Compared to hiking, the risk of knee and cruciate ligament injuries, particularly of the anterior cruciate ligament (ACL), is significantly higher in skiing. Data from the latest survey by the Swiss Council for Accident Prevention (bfu.ch) showed that on average, 22,260 people get injured while hiking every year, and approximately 51,700 while skiing [2] - 30.9% of these injuries affect the knee joint [3], especially the anterior cruciate ligament. We witness this repeatedly every winter at BodyLab, Osteopathy and Physiotherapy, Rehabilitation and Training in Zurich.
Outside the ski season, football is one of the most widespread recreational sports in the world, including in Switzerland. Injuries to the lower extremities, and specifically the knee, also frequently occur in this sport, as studies confirm [4].
Various studies estimate that up to about 17.6% of football-related injuries that present to emergency departments affect the knee [5,6,7,8,9].
The majority of these football injuries resulted from non-contact mechanisms (85% of injuries without the involvement of an opponent), although contact injuries in tackles are naturally also possible [10,11]
ACL Anatomy and Function
The ACL is one of the most important ligament structures around and within the knee joint. Together with the posterior cruciate ligament (PCL) and the collateral ligaments, it contributes to the stabilization of the knee joint by spanning from the posterior end of the thigh bone to the anterior portion of the tibia head.
The tight connective tissue of the ACL mainly consists of type-I collagen fibers and is subdivided by type-III fibers into two primary bundles with different functions. The anterior-medial part is on average 36mm long and gets tensioned at 30° knee flexion. The main task of the anterior-medial bundle is stabilization against anterior tibial translations, i.e., it prevents the tibia from sliding forward.
The posterior-lateral bundle is approximately 18mm long and gets tensioned at less than 30° flexion. This part of the ACL, along with other ligament structures, serves the stabilization of rotational forces in the knee joint.
The ACL is not just a passively stabilizing structure within the knee joint. Based on the Pacinian corpuscles and Ruffini endings as well as other free nerve endings in the insertion area of the band, vibration sensations, stretch, and shear forces are perceived. This information supports the perception of joint position to generate adjusted muscular responses. In the case of the ACL, this means increased tension in the hamstrings to reduce anterior tibial translation.
Injury Mechanisms
An ACL injury can be triggered by contact as well as non-contact mechanisms.
Typical mechanisms and the most common cause of an ACL injury is through a combination of twisting and buckling trauma of the knee joint under load (vigorous valgus buckling of the knee (inwards) with slight knee flexion and combined rotation).
Injury Mechanisms in Football
Due to the studded shoes, which fix the foot to the ground in the grass, enormous uncontrolled forces can affect the knee. 85% of all ACL injuries in football result from non-contact situations [10,11]. Video analyses of professional footballers also show an increased risk of ACL injury from non-contact situations within the first 9 minutes of the game, as well as that the risk for injuries is greater during defensive and tackling activities [12,13,14].
Injury Mechanisms in Skiing
Through video analyses of elite alpine skiers after ACL rupture, Bere et al. identified three accident mechanisms [15]:
Slip-Catch
Dynamic Snowplow
Landing after jumps
Slip Catch
In almost 50% of all cases, a so-called slip-catch mechanism was the cause. In this scenario, the skier lost snow contact with the outer ski during a turn. After regaining snow contact, the ski caught, resulting in increased knee flexion, internal tibia rotation, and valgus buckling of the knee. This mechanism often leads to injury of the medial meniscus and medial collateral ligament – known as the Unhappy Triad.
Dynamic Snowplow
Another incident mechanism (15%) was the dynamic snowplow. Here, the skier was thrown off balance due to leaning back and imbalance, causing the less-loaded leg to drift away and the skier ending up in a split position. The ski rolled from the outer edge onto the inner edge, caught in the snow, bringing the knee into a valgus position and internal tibial rotation.
Landing after Jumps
The last mechanism (20%) occurred on landing after jumps. Here, the skier was out of balance, with the body weight being pushed backwards. When trying to regain balance, the ACL was exposed to high loads due to a combination of increased knee compression and significant anterior tibia shift (movement of the tibia forward relative to the femur).
Considering that much larger forces are at work for professional skiers due to speed and equipment, these mechanisms cannot be completely transferred to amateur athletes. However, the individual movement factors (strong flexion, valgus position, and tibial internal rotation) are the same.
For amateur athletes, so-called forward twisting falls are the most common accident mechanism, at 54% [16]. This involves a valgus position and internal tibial rotation after catching the ski edge while turning.
The second most frequent incident mechanism is called backward twisting fall or phantom foot [6]. In 26% of all cases, this rupture mechanism occurs. Here, the center of gravity shifts towards the back on one leg due to the pelvis dropping (increased knee flexion), whether during a turn or after a fall, and the weight is pressed onto the end of the ski. In combination with rather stiff boots, the tibia is pushed forward, and the ACL tears.
Knee traumas often include accompanying injuries (see Unhappy Triad, medial collateral ligament injuries, meniscus injuries, cartilage damage, secondary osteoarthritis).
Contributing Factors / Equipment
With the emergence of carving skis, it showed that the increased aggressiveness of the ski due to shaping, in combination with hard ski boots, leads to more injuries of the ACL [15]. Smaller turns can be made, and more speed is generated. This requires more strength from the skier and leads to more difficult control of the equipment. Due to the stiffness of the ski boots, the tibia is pushed anteriorly with backward weight shifts, placing more stress on the ACL. Furthermore, video analyses showed that in 100% of all slip-catch cases, the binding did not release or released too late [15].
Biomechanics in the Knee after ACL Rupture
Primarily, there is a combination of anterior knee instability with anterior tibial translation and rotational instability. This affects the entire biomechanics of the knee. The rotation center in the knee joint shifts medially. This leads to increased stress on the collateral ligaments (medial and lateral collateral ligaments) and the meniscus horns, which also places more stress on the cartilage surfaces. For example, the medial collateral ligament is subjected to up to 140% more stress, and the posterior outer structures must perform up to 400% more [17]. This is observed while walking with an extended flexion during the stance phase and increased anterior tibial translation towards the end of the stance phase [17].
Surgery or Conservative Treatment
After an ACL injury, there are generally two options - surgical reconstruction of the ACL followed by physiotherapy, or conservative treatment with physiotherapeutic measures - with the option for surgery if needed.
In Switzerland, the Swiss Medical Board has also addressed this issue (and published a fact sheet [19] on it): After a thorough literature search, the panel concluded that satisfactory results can be achieved with both surgical and conservative approaches - earlier physical activities can usually be resumed. Studies showing outcome assessments after one and five, or up to 15 years after an ACL injury indicate no significant differences when comparing conservative treatment with surgery [18].
In the conservative treatment, there is the possibility that if persistent knee instability remains, surgery can still be performed during the course of therapy without losing time.
Conclusion: "When choosing the treatment method, the respective advantages and disadvantages (namely the risk of surgery) must be carefully examined; if necessary, a second opinion should be obtained." [19]
Conservative Therapy
The goal of conservative therapy and physiotherapy after an ACL injury is to restore physiological mobility, as well as stabilization of the knee joint and leg axis. Following injury, the body responds with certain biological processes (inflammatory phase, proliferation phase, remodeling phase), which a physiotherapist must consider to enable the fastest possible healing and restoration of function.
According to the healing phases, short-, medium-, and long-term goals are set for therapists and patients. In the first 1-2 weeks (inflammatory phase), reduction of swelling and pain relief through passive measures such as lymphatic drainage and movement without weight bearing are certainly central themes of treatment [20].
During the proliferation phase (3rd-6th week), the focus is on regaining mobility, progressively increasing load (potentially reducing crutch use, from partial weight-bearing to full weight-bearing), improving stability, and strengthening muscular deficits in closed kinetic chains. This is done through passive and active exercises [20].
From the 7th post-traumatic week (remodeling phase), the complete restoration of leg axis stability (pelvis-leg axis) and muscle building with strengthening exercises in the gym are central. The goal is to be symptom-free in everyday life and return to previous sports activities [20].
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References
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Sport Switzerland 2014: Sports activity and interest of the Swiss population. Magglingen: Federal Office for Sport FOSPO.
[2] Injured by sport and age, Ø 2010–2014
[3] Injury location in UVG-insured by sport (per 100 injured), Ø 2008–2012
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[12] Anterior cruciate ligament injury in elite football: a prospective three-cohort study.
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Grassi A, Smiley SP, Roberti di Sarsina T, et al.
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[17] Rupture of the anterior cruciate ligament. Biomechanical impact on the knee joint
PD Dr. M. Herbort, Prof. Dr. C. Fink:
in: Arthroscopy (01/2015)
[18] Review Technical Report "Rupture of the ACL: surgical or conservative treatment?"
[19] Fact Sheet Rupture of the anterior cruciate ligament: surgical or conservative treatment?
[20] ACL Rupture-Therapy, AGA-Knee-Ligament-Committee
Cover Image Credit
Kari Stammen, Human Knee Anatomy, labeled as public domain, details on Wikimedia Commons
