When is acl taut

With both internal and external rotation, the ACL tightens so that it may operate as a major restraint against rotational moments acting about the knee joint [ 7 ]. Fibre arrangement of the anteromedial a, black arrow and the posterolateral b, white arrow during extension a and flexion b of the knee. Anatomical footprint of the ACL.

Like any other ligamentous structure, the biomechanical properties of the ACL are determined by the geometry of the ligament as well as the tensile characteristics of both ligament midsubstance and the ligament-to-bone insertion site. Basically, they can be characterised by the relationship between ligament length and ligament tension, which can be determined when simultaneously measuring load and the corresponding elongation during experimental uniaxial tensile testing.

The resultant load-elongation curve can be divided into four distinct regions according to the structural properties of the ACL. The toe region is followed by a quasilinear region where collagen fibres reversibly deform. The slope of the linear region allows for reproducible determination of ligament stiffness measured in Newtons per millimetre and corresponds to the loads acting on the ACL during daily activities.

In the intact knee, both the toe region and the linear region of the ACL loading curve will allow the tibia to translate anteriorly for 3—5 mm during knee motion as well as during an anterior drawer manoeuvre. With additional loading, the slope of the load-elongation curve decreases yield-point as plastic deformation of the collagen fibres occurs. Finally, the curve reaches the ultimate load, which is described as failure of the bone-ligamentbone complex. It may be derived from the load-elongation curve, that applying high loads to a ligament will increase the stiffness and may therefore sufficiently restrict excessive joint motion when high external loads are applied.

In vivo , cyclic loading of the ACL will cause gradual creep and relaxation, which results in increased knee laxity after physical activity. However, it will return to the original stiffness after a period of rest. The parameters derived during experimental ligament loading are considered to be essential to understanding ligament function and evaluating the appropriateness of different graft materials and fixation techniques commonly used in ACL reconstruction [ 26 — 28 ].

In addition, visual or apperative control of the mode of failure during tensile testing allows identification of either graft slippage or deterioration of graft material under mono- or polycylic loading conditions, thus indicating what amount of graft tension loss should be expected during the postoperative rehabilitation period.

Estimations of ACL forces during activities of daily living calculated by Morrison revealed that ACL loads of N may be expected during normal level walking, while descending stairs generated N of in situ force due to the activation of the knee extensor apparatus [ 29 — 31 ]. In contrast, ascending stairs as well as ascending or descending a ramp generated in situ forces below N. While estimation of in vivo ACL forces during normal activity have been subject to computational analyses, the biomechanical properties of the native ACL have extensively been analysed in ex vivo studies.

Focusing on the ACL force and ligament deformation during anterior tibial translation, Sakane et al. This suggests that the role of the posterolateral bundle in response to anterior tibial loads near extension may be of more importance than was previously thought Fig.

Adapted from [ 23 ]. Performing tensile testing of the bone-ligament-bone complex, Woo et al. They were also able to demonstrate that both the ultimate failure load and the linear stiffness significantly decreased with age and with the axis of loading. Given this data, Noyes et al. ACL injuries commonly occur during sport activities with sudden stresses applied to the knee joint while the tibia is in contact with the ground.

Typically, the ACL is torn in a non-contact deceleration situation that produces valgus and internal rotational moments on the knee joint that begins to flex from near extension.

This usually occurs in high-risk pivoting sports when the athlete lands on the leg and starts rotating to the opposite direction [ 37 ]. Active quadriceps pull is considered to play an important role in the pathomechanism of ACL injury. Reflective eccentric quadriceps contraction accompanied by apparent weakness of the hamstring muscles allows the extensor mechanism to strain the ACL during anterior tibial translation. This mechanism can be observed during jump stop landings.

Less frequently, direct contact injuries occur as a result of extensive valgus stress or hyperextension of the knee joint. Those concomitant lesions are reported to significantly influence the long-term functional outcome. Additionally, in both isolated and combined ACL injuries, minor or major bruising of chondral and subchondral structures may be present. Histologic analyses of bruised bone performed by Johnson et al. Disruption of the ACL inevitably results in alterations in knee kinematics as a transfer of loads can be effective only if the joint is mechanically stable.

ACL insufficiency causes deterioration of the physiologic roll-glide mechanism of the femorotibial joint and results in an increased anterior tibial translation as well as an increased internal tibial rotation. In the advent of muscular fatigue or deficient neuromuscular control, the patient will experience this combined anterior and rotatory instability as a subluxation of the femorotibial joint.

According to the concept of primary and secondary restraints, failure of a primary restraint will cause recruitment of secondary structures in order to resist external forces and to stabilise joint motion. The increase in loads applied to secondary structures may render them more susceptible to degeneration or secondary failure [ 44 — 46 ].

Current research supports the concept that under observance of several key factors, arthroscopically assisted ACL reconstruction done with a biologic autograft significantly improves the stability and function of the knee in most patients. The key factors significantly influencing the functional outcome are:. Currently recommended graft materials for ACL reconstruction are biologic autografts and, although rarely available in Western Europe, allografts [ 48 ].

Although a surgeon may prefer one specific substitute for reconstruction, modern knee surgery requires individual concepts and a variability of treatment options. In their metaanalysis on the functional outcome of patellar tendon and hamstring tendon ACL reconstructions, Freedman et al. Several other studies confirmed no significant difference in functional parameters and subjective results obtained at follow-up when comparing patellar tendonbone and hamstring tendon grafts [ 57 ].

From a biomechanical point of view, no graft material commonly used has ultimate failure strength or stiffness comparable to the native ACL. Although ultimate failure loads of the native bone-patellar tendon-bone complex, a quadrupled hamstring tendon complex, or the quadriceps tendon-bone complex average , and N, respectively, and consequently exceed the values reported for the native ACL, graft harvest and artificial graft fixation into bone significantly decreases both the ultimate strength and the linear stiffness [ 58 — 60 ].

Patellar tendonbone grafts should be used for young patients and highdemand athletes who prefer early return to high-level activities, while hamstring tendons are advantageous when a large skin incision or anterior knee pain should be avoided.

Quadriceps tendon grafts should primarily be used for revision surgery as they are difficult to harvest and size and location of donor-site scar are disadvantageous [ 61 ]. The key to proper postoperative knee function is to restore the physiologic roll-glide mechanism of the femorotibial joint, and thus avoid both increased anterior displacement and pathologic patterns of knee rotation.

Investigating the anterior-posterior stability of the knee after ACL reconstruction, Rupp et al. Fu et al. Consequently, avoiding nonphysiological strain patterns of a ligament graft throughout the functional range, which also avoids graft failure and any limitation in knee motion, has been emphasised for successful reconstruction [ 7 ].

A proposed isometric bone tunnel placement i. Malposition of the femoral tunnel and resulting functional consequences [[ 65 ]. Hefzy et al. They reported that altering the location of the femoral bone tunnel had a much greater effect on the length of the substitute than did altering the tibial attachment site did. They also pointed out that the area in which tunnel placement resulted in length changes of the graft of less than 2 mm throughout the range of motion was smaller than the cross-sectional area of grafts commonly used for reconstruction of the ACL.

The resulting inhomogenous tension patterns within a graft would therefore not support the concept of isometric graft placement. Csizy and Friederich noted that with an arthroscopic view of the knee joint, the femoral tunnel is most susceptible to being placed anterior to the anatomic ACL footprint, thus resulting in excessive graft tension during knee flexion and correlating well with poor functional outcomes [ 65 , 67 ].

Varying the femoral tunnel position between the anatomic ACL footprint and the most isometric position, Musahl et al. However, they concluded that anatomic graft placement resulted in kinematics closer to the normal knee joint than did a tunnel placement for best isometry. Several methods for measuring femoral graft position in postoperative radiographs have been introduced, hence enabling visual control of bone tunnel placement when using intraoperative flouroscopic imaging [ 68 — 72 ] Fig.

Investigations performed by Klos et al. Probably the most popular technique for radiographic bone tunnel measurement is the quadrant method described by Bernard et al.

Although commonly used to identify the anatomic ACL origin, it is reliable only when the projection of the femoral condyles perfectly overlap in lateral radiographs of the knee. Radiographic analysis of femoral bone tunnel placement. Tibial bone tunnel placement has been reported to be less sensitive with respect to postoperative knee kinematics, however, may cause graft impingement or unphysiologic loading patterns when malplaced [ 73 — 75 ].

The tibial tunnel should be placed within the posterior half of the native ACL footprint, which will allow the anterior fibres of the graft to run parallel with the intercondylar roof in full extension Fig. Anterior placement of the tibial tunnel as well as a vertically oriented intercondylar roof in the sagittal plane will either initially subject the graft to roof impingement or secondarily will cause roof impingement with contraction of the quadriceps muscle. It has been recommended in the literature to leave 6 mm of clearance between the anterior aspect of the graft and the intercondylar roof during extension of the knee.

In the frontal plane, the position of the femoral tunnel along the intercondylar notch can be described by the angular position of numerals on the face of a clock.

However, Markolf et al. Hence, they demonstrated that the biomechanical consequences of varying femoral tunnel placement in the frontal plane were less critical than varying the anteroposterior position. In order to obey the complex structure of the intact ACL, several authors have claimed ACL reconstruction to be more anatomical when both the anteromedial and the posterolateral bundle were replaced independently [ 84 — 87 ].

Yagi et al. They demonstrated that under combined anterior, internal tibial and valgus torque, knee stability using a double-bundle technique was superior to a single-bundle reconstruction technique. There is agreement among ACL surgeons that double-bundle ACL reconstruction is a demanding procedure that currently may only be performed in experienced trauma centres.

The tension applied to the graft before final graft fixation significantly influences the kinematics of the knee joint and the long-term ability of a graft to stabilise the knee joint. A low initial graft tension will not provide adequate joint stability, while excessive initial graft tension will restrain range of motion and is susceptible of early graft failure. Yoshia et al. They demonstrated that anteroposterior knee stability did not significantly differ between grafts fixed at 1 N and 39 N three months after surgery, however, knee joints with higher initial graft tension showed evidence of degenerative cartilage lesions.

In a goat model, Abramowitch et al. In a prospective randomised trial, Kim et al. In accordance, in vitro studies on the course of graft tension have shown that both the patellar tendon-bone graft and the hamstring tendon graft lose their initial tension when being cyclically loaded [ 91 , 92 ]. There is a lack of scientifically based data on the clinical impact of initial graft tension as follow-up studies so far have failed to prove that variations of graft tension resulted in clinical symptoms after ACL reconstruction.

At present, the amount of tension that should be applied to a graft prior to fixation has yet to be precisely defined. Excessive tension as well as loose fixation should therefore be controlled intraoperatively by testing range of motion and anterior knee stability under arthroscopic visualisation. Although the influence of the viscoelastic behaviour of ACL replacements so far has not been entirely characterised, the viscoelastic creep or relaxation may contribute to a decrease in graft tension over time.

Consequently, several authors have emphasised that grafts should be cyclically preconditioned prior to implantation in order to decrease the viscoelastic elongation behaviour during rehabilitation [ 93 , 94 ]. The subject of graft preconditioning remains controversial as the preconditioning protocols described in the literature are not commonly applicable to the surgical procedure and may not be as beneficial as suggested by ex vivo biomechanical studies [ 93 — 95 ].

Most fixation devices currently used rely on linkage material between the graft substance and the bone, thus influencing graft healing and altering the initial biomechanical properties of a graft material.

Generally, fixation devices distant to the joint line i. For this reason, I have my patients begin working on motion with their physical therapist by day three post-surgery. Here is a breakdown of the typical postoperative recovery from ACL reconstruction surgery.

Fortunately, after the long rehabilitation efforts, the vast majority of patients regain full strength, motion and stability of the knee and can return to their sport of choice after ACL reconstructive efforts. Bryan M. August 12, Saltzman, MD While many anterior cruciate ligament ACL injuries heal with time and proper rest, more severe ligament tears may need to be repaired surgically.

Which patients are candidates for ACL reconstruction surgery? How does a patient prepare for ACL reconstruction surgery? What is an ACL graft? How is ACL reconstruction surgery performed?

Arthroscopic image of torn ACL fibers Arthroscopic image of reconstructed ACL using the tendon from the front of the knee What is the recovery process like? How long will it take A Patient to return to sports?

Share on Twitter Share on Facebook. Author information Article notes Copyright and License information Disclaimer. Corresponding author. Tel: , Fax: , rk. Received Jan 29; Accepted Jun 5. This article has been cited by other articles in PMC.

Open in a separate window. Schematic drawing of ACL bundles in flexed knee. Illustrations of anteroposterior view of knee showing technique for single bundle A and double bundle reconstruction B of ACL.

Surgical Navigation Terminology Since surgery is performed with the knee in flexion, surgical terminology differs from anatomical terminology Fig.

Terms and directions used in anatomy and radiology A and terms and directions used in surgery B. Measurements of Tunnel Positions: Imaging-Based Methods Malpositioning of the bone tunnels is considered as one of the most common technical errors in ACL reconstruction Normal femoral tunnel position. Tibial Tunnel The single-bundle graft is required to provide both anterior-posterior and rotational pivotal stability.

Normal tibial tunnel position. Angular measurement of tibial tunnel. Normal femoral and tibial tunnels on multiplanar reformat CT images. Abnormalities in the Early Post-Operative Imaging Deviations from the Optimal Location When femoral tunnel placement is too shallow and too high, the graft is taut in flexion.

Schematic drawing of kinematics of graft on flexion A and extension B. Example of tibial tunnel positioned too anteriorly. Abnormal Findings at Fixation Site Hamstring grafts are fixed with a device like a button to suspend it at the femur and a screw for instance bioabsorbable screw to fix it in the tibia. Migration of button style extra-cortical fixation device.

Gap between fixation device and bone cortex. Migration of bioabsorbable interference screws. Tunnel Widening Tunnel enlargement after ACL reconstruction is a well-known phenomenon that predominantly occurs during the first six months after surgery, and represents a potential problem for revision surgery Tunnel widening.

Intramuscular Location of a Screw Tip Protrusion of screw tip into calf muscles can cause indentation and popliteal area pain. Post-operative CT scan shows intramuscular location of screw tip.

Patient had popliteal area pain 3 months after surgery and fixator was removed. Divergence Interference screws provide the most secure fixation in the immediate postoperative period, and the optimal orientation of the screw within the tunnel for maximum fixation strength is parallel to the graft.

Miscellaneous Any case resulting in intra-operative bone fractures can be fixated during the operation or can heal spontaneously without complication. Summary Given the increasing number of patients undergoing ACL reconstruction, it is imperative for radiologists to be familiar with these procedures and the associated abnormalities.

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