Anatomic relation between the posterior cruciate ligament and the joint capsule

A modified anatomical posterior cruciate ligament reconstruction technique using the posterior septum and posterior capsule as landmarks to position the low tibial tunnel

BACKGROUND

Lowering the exit position of the tibial tunnel can improve the clinical efficacy of posterior cruciate ligament (PCL) reconstruction, however, there is no unified positioning standard. This study aimed to use novel soft tissue landmarks to create a low tunnel.

METHODS

A total of 14 human cadaveric knees and 12 patients with PCL injury were included in this study. Firstly, we observed the anatomical position between the PCL, posterior septum, and other tissue, and evaluated the relationship between the center of the low tibial tunnel (SP tunnel) and posterior septum and distal reflection of posterior capsule, and using computed tomography (CT) to evaluate distance between the center of the SP tunnel with bony landmarks. Then, evaluated the blood vessels content in the posterior septum with HE staining. Finally, observed the posterior septum and distal reflection of the posterior capsule under arthroscopy to explore the clinical feasibility of creating a low tibial tunnel, and assessed the risk of surgery by using ultrasound to detect the distance between the popliteal artery and the posterior edge of tibial plateau bone cortex.

RESULTS

In all 14 cadaveric specimens, the PCL tibial insertions were located completely within the posterior medial compartment of the knee. The distance between the center of the SP tunnel and the the articular surface of tibial plateau was 9.4 ± 0.4 mm. All SP tunnels retained an intact posterior wall, which was 1.6 ± 0.3 mm from the distal reflection of the posterior capsule. The distances between the center of the SP tunnel and the the articular surface of tibial plateau, the champagne glass drop-off were 9.2 ± 0.4 mm (ICC: 0.932, 95%CI 0.806-0.978) and 1.5 ± 0.2 mm (ICC:0.925, 95%CI 0.788-0.975) in CT image. Compared with the posterior capsule, the posterior septum contained more vascular structures. Last, all 12 patients successfully established low tibial tunnels under arthroscopy, and the distance between the posterior edge of tibial plateau bone cortex and the popliteal artery was 7.8 ± 0.3, 9.4 ± 0.4 and 7.4 ± 0.3 mm at 30°, 60° and 90° flexion angels after filling with water and supporting with shaver in posterior-medial compartment of knee joint.

CONCLUSIONS

A modified low tibial tunnel could be established in the PCL anatomical footprint by using the posterior septum and posterior capsule as landmarks.

Relationship of the Cruciate and Meniscofemoral Ligaments with the Knee Osteology. An Anatomical Study *

Objective  To analyze the dimensions of the posterior cruciate ligament (PCL), anterior cruciate ligament (ACL), the presence of meniscus-femoral ligaments MFLs in human knees, and the correlation with the dimensions of the knee skeleton.

Methods  Anatomical study on 29 specimens of human knees in which we measured the length and width of the cruciate and meniscus-femoral ligaments and the dimensions of femoral and tibia condyles and the femoral notch. The ACL length was calculated with different degrees of knee flexion. The relationship between the ligaments and bone dimensions were analyzed.

Results  The length of the ACL and the PCL were similar. Posterior MFL was more frequent and longer than the anterior MFL. We found the posterior MFL in the 72.41% of the knees and anterior MFL in 20.69%. The ACL presented 30% of its maximum length up to 60°, approximately half of its length between 90° and 120°, reaching its maximum length at 170°. We found a strong correlation between the length of the ACL and that of the PCL ( p  = 0.001). However, the lengths of the ACL and PCL were not related with the bone dimensions.

Conclusion  We have found no correlations between the cruciate and MFLs and the anatomical dimensions of the intercondylar notch and the proximal tibia and distal femur. The presence of the posterior MFL was more frequent and longer than that of the anterior ligament.

Prevalence, Biomechanics, and Pathologies of the Meniscofemoral Ligaments: A Systematic Review

Purpose

To systematically review the literature to examine current understanding of the meniscofemoral ligaments (MFLs), their function, their importance in clinical management, and known anatomical variants.

Methods

A systematic review was conducted following Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines using PubMed, EMBASE, and Cochrane databases. Studies were included if they reported on the biomechanical, radiographic, or arthroscopic evaluation of human MFLs, or if they reported on an anatomical variant. These were then categorized as cadaveric, radiographic, or clinical. Biomechanical, radiographic, patient-reported, and functional outcomes data were recorded.

Results

Forty-seven studies were included in the qualitative analysis, and 26 of them were included in the quantitative analysis. Of these, there were 15 cadaveric, 3 arthroscopic, and 9 radiographic studies that reported on the prevalence of MFLs. Overall, when looking at all modalities, the presence of either the anterior or posterior MFL (aMFL, pMFL) has been noted to be 70.8%, with it being the aMFL 17.4% and the pMFL 40.6%. The presence of both ligaments occurs in approximately 17.6% of individuals. Eleven reported on mean MFL length and thickness. When evaluating mean length in both men and women, the aMFL has been reported between 21.6 and 28.3 mm and the pMFL length in this population is between 23.4 and 31.2 mm. Five reported on cross-sectional area. Nine additional papers report anatomical variants.

Conclusions

This review shows that there continues to be a variable incidence of MFLs reported in the literature, but our understanding of their function continues to broaden. A growing number of anatomic and biomechanical studies have demonstrated the importance of the MFLs in supporting knee stability. Specifically, the MFLs serve an important role in protecting the lateral meniscus and augmenting the function of the posterior cruciate ligament.

Clinical Relevance

Our findings will aid the clinician in both identifying and treating pathologies of the meniscofemoral ligaments.

Clinical Anatomy of the Posterior Meniscofemoral Ligament of Wrisberg: An Original MRI Study, Meta-analysis, and Systematic Review

Background

The posterior meniscofemoral ligament (pMFL) of Wrisberg attaches to the posterior horn of the lateral meniscus and the lateral intercondylar aspect of the medial femoral condyle and passes posteriorly to the posterior cruciate ligament (PCL). The pMFL plays a role in recovery after PCL injuries and offers stability to the lateral meniscus, promoting normal knee function.

Purpose/Hypothesis

The aim of the magnetic resonance imaging (MRI) arm of this study was to evaluate the prevalence of the pMFL in Polish patients. The purpose of the systematic review and meta-analysis was to evaluate the clinical relevance of the pMFL in knee surgery. It was hypothesized that extensive variability exists in reports on the prevalence, function, and clinical significance of the pMFL.

Study Design

Cross-sectional study and systematic review; Level of evidence, 3.

Methods

A retrospective MRI investigation was conducted on 100 randomly selected lower limbs of Polish patients (56 male, 44 female) performed in 2019 to determine the prevalence of the pMFL. Additionally, an extensive literature search of major online databases was performed to evaluate all reported data on the pMFL. Assessments of article eligibility and data extraction were completed independently by 2 reviewers, and all disagreements were resolved via a consensus. A quality assessment of the included articles was performed using the Anatomical Quality Assessment tool.

Results

In the MRI arm of this study, the pMFL was observed in 73 of the 100 limbs. In the meta-analysis, 47 studies were included, totaling 4940 lower limbs. The pooled prevalence of the pMFL was found to be 70.4% (95% CI, 63.4%-76.9%); the mean length was 27.7 mm (95% CI, 24.8-30.5 mm) and the mean widths were 4.5, 6.1, and 4.1 mm for the meniscal and femoral attachments and midportion, respectively. The mean pMFL thickness was 2.3 mm (95% CI, 1.8-2.7 mm).

Conclusion

Despite the variability in the literature, the pMFL was found to be a prevalent and large anatomic structure in the knee joint. The shared features of this ligament with the PCL necessitate the consideration of its value in planning and performing arthroscopic procedures of the knee.

Knee and Leg Injuries

Knee and leg injuries are extremely common presentations to the emergency department. Understanding the anatomy of the knee, particularly the vasculature and ligamentous structures, can help emergency physicians (EPs) diagnose and manage these injuries. Use of musculoskeletal ultrasonography can further aid EPs through the diagnostic process. Proper use of knee immobilizers can also improve long-term patient outcomes.

[Meniscal root lesions: clinical relevance and treatment]

Meniscal roots play an important role in load distribution of the tibiofemoral knee joint. Thus, meniscal root tears are severe injuries of the knee, which significantly expose cartilage to increased stress. Two entities are distinguished: (i) acute root tears that mainly affect the posterolateral root and often occur with ruptures of the anterior cruciate ligament; and (ii) chronic, degenerative root tears that mainly affect the posteromedial root. For diagnosis of both entities, the patient's medical history, a knee examination and especially MRI play key roles. The treatment of choice is in general transtibial fixation of the torn root, which leads to an increased clinical improvement and a decrease of the progression of arthritis.

Posterolateral portal tibial tunnel drilling for posterior cruciate ligament reconstruction: technique and evaluation of safety and tunnel position

PURPOSE

To evaluate the safety for neurovascular structures and accuracy for tunnel placement of the posterolateral portal tibial tunnel drilling technique in posterior cruciate ligament (PCL) reconstruction.

METHODS

Fifteen fresh-frozen human cadaveric knees were used. The tibial tunnel for the PCL was created using a flexible reamer from the posterolateral portal. Then, the flexible pin was left in place, and the distance from the posterolateral portal, the flexible pin, and the tibial tunnel to the peroneal nerve and popliteal artery was measured. Additionally, the distance between the tibial tunnel and several landmarks related to the PCL footprint was measured, along with the distance from the exit point of the flexible pin to the superficial medial collateral ligament and gracilis tendon.

RESULTS

The peroneal nerve and the popliteal neurovascular bundle were not damaged in any of the specimens. The median (range) distance in mm from the peroneal nerve and popliteal artery to the posterolateral portal and flexible pin was: 52 (40-80) and 50 (40-61), and 35 (26-51) and 22 (16-32), respectively. The median (range) distance from the tibial tunnel to the popliteal artery was 21 mm (15-38). The tibial tunnel was located at a median (range) distance in mm of 3 (2-6), 6 (3-12), 5 (2-7), 4 (1-8), 9 (3-10), 10 (4-19), and 19 (6-24) to the champagne-glass drop-off, lateral cartilage point, shiny white fibre point, medial groove, medial meniscus posterior root, lateral meniscus posterior root, and posterior aspect of the anterior cruciate ligament, respectively.

CONCLUSIONS

The posterolateral portal tibial tunnel technique is safe relative to neurovascular structures and creates an anatomically appropriate tibial tunnel location. The clinical relevance of study is that this technique may be safely and accurately used in PCL reconstruction to decrease the risk of neurovascular damage (avoid use of a posteriorly directed pin), avoid the use of intraoperative fluoroscopy, and avoid the sharp turn during graft passage.

Relationship Between the Middle Genicular Artery and the Posterior Structures of the Knee: A Cadaveric Study

Background:

The middle genicular artery (MGA) is responsible for the blood supply to the cruciate ligaments and synovial tissue. Traumatic sports injuries and surgical procedures (open and arthroscopic) can cause vascular damage. Little attention has been devoted to establish safe parameters for the MGA.

Purpose:

To investigate the anatomy of the MGA and its relation to the posterior structures of the knees, mainly the posterior capsule and femoral condyles, and to establish safe parameters to avoid harming the MGA.

Study Design:

Descriptive laboratory study.

Methods:

Dissection of the MGA was performed in 16 fresh, unpaired adult human cadaveric knees with no macroscopic degenerative or traumatic changes and no previous surgeries. The specimens were meticulously evaluated with emphasis on preservation of the MGA. The distances from the MGA to the medial and lateral femoral condyles were measured. The Mann-Whitney test was used for statistical analysis.

Results:

In all specimens, the MGA emerged from the anterior aspect of the popliteal artery, distal to the superior genicular arteries, and had a short distal trajectory toward the posterior capsule where it entered proximal to the oblique popliteal ligament. The artery lay in the midportion between the condyles. The distance between the posterior aspect of the tibia and the point of entry of the MGA into the posterior joint capsule was 23.8 ± 7.3 mm (range, 14.72-35.68 mm). There was no correlation between an individual’s height and the distance of the entrance point of the MGA into the posterior joint capsule to the posterior superior corner of the tibia.

Conclusion:

The middle genicular artery lies in the midportion between the medial and lateral femoral condyles.

Clinical Relevance:

This knowledge is important for the preservation of the blood supply during posterior knee surgical procedures and to settle a secure distance between the posterior aspect of the tibia and the MGA input. This could decrease and prevent iatrogenic vascular injury risk to the MGA.

Arthroscopic Posterior Knee Capsulotomy for a Fixed Flexion Contracture in a Ten-Year-Old Boy Due to a Hypertrophied Ligamentum Mucosum: A Case Report

CASE

Knee flexion contractures are difficult to treat effectively. We present the case of a ten-year-old boy with a flexion contracture due to a hypertrophied ligamentum mucosum treated with an arthroscopic posterior capsular release. The patient obtained full extension and maintained it at the time of final follow-up.

CONCLUSION

This case represents a unique cause of an impinging ligamentum mucosum and provides anecdotal support to the concept of an arthroscopic posterior capsular release as a safe and effective option for flexion contractures.

A simple method to minimize vascular lesion of the popliteal artery by guidewire during transtibial posterior cruciate ligament reconstruction: a cadaveric study

PURPOSE

To compare the outside-in transtibial lateral and medial approaches for posterior cruciate ligament (PCL) reconstruction regarding the guidewires and popliteal artery integrity.

METHODS

Twenty-two human cadaveric knees were used. A PCL tibial aimer was arthroscopically placed within the PCL footprint through the anteromedial portal for the medial approach and through the anterolateral portal for the lateral approach. For the medial approach, the drill guide was introduced through the anteromedial tibial cortex and the guidewire was advanced with the reamer beyond the posterior tibial cortex. For the lateral approach, the drill guide was introduced through the anterolateral tibial cortex and the guidewire was advanced with the reamer beyond the posterior tibial cortex. After this, the knee was dissected. The depth distance (DD) was defined as the distance between the popliteal artery and the tibial posterior cortex projected at the tibial level at which the guidewire intersected or passed by the artery. The guidewire travel distance was calculated as the distance the guidewire had to advance beyond the tibial cortex to intersect the popliteal artery or pass by it.

RESULTS

With the medial approach, the popliteal artery was intersected in all knees with a mean DD of 12.20 mm and a mean guidewire travel distance of 15.90 mm. With the lateral approach, the popliteal artery was not intersected in any knee; its mean medial distance from the artery was 4.8 mm at a DD of 10.05 mm. There was a significant difference in the popliteal artery intersection incidence and DD between both groups (P < .0001 and P = .0003, respectively).

CONCLUSIONS

The transtibial lateral approach for PCL reconstruction was a safer method than the medial approach regarding popliteal artery injury by a guidewire.

CLINICAL RELEVANCE

This study presents a slight modification of the most frequently used PCL reconstruction technique, intending to minimize guidewire injury to the popliteal artery.

Posterior cruciate ligament reconstruction by means of tibial tunnel: anatomical study on cadavers for tunnel positioning

OBJECTIVE

to determine the reference points for the exit of the tibial guidewire in relation to the posterior cortical bone of the tibia.

METHODS

sixteen knees from fresh cadavers were used for this study. Using a viewing device and a guide marked out in millimeters, three guidewires were passed through the tibia at 0, 10 and 15 mm distally in relation to the posterior crest of the tibia. Dissections were performed and the region of the center of the tibial insertion of the posterior cruciate ligament (PCL) was determined in each knee. The distances between the center of the tibial insertion of the PCL and the posterior tibial border (CB) and between the center of the tibial insertion of the PCL and wires 1, 2 and 3 (CW1, CW2 and CW3) were measured.

RESULTS

in the dissected knees, we found the center of the tibial insertion of the PCL at 1.09 ± 0.06 cm from the posterior tibial border. The distances between the wires 1, 2 and 3 and the center of the tibial insertion of the PCL were respectively 1.01 ± 0.08, 0.09 ± 0.05 and 0.5 ± 0.05 cm.

CONCLUSION

the guidewire exit point 10 mm distal in relation to the posterior crest of the tibia was the best position for attempting to reproduce the anatomical center of the PCL.

Tibial tunnel placement in posterior cruciate ligament reconstruction: a systematic review

PURPOSE

Reconstruction of the posterior cruciate ligament (PCL) yields less satisfying results than anterior cruciate ligament reconstruction with respect to laxity control. Accurate tibial tunnel placement is crucial for successful PCL reconstruction using arthroscopic tibial tunnel techniques. A discrepancy between anatomical studies of the tibial PCL insertion site and surgical recommendations for tibial tunnel placement remains. The objective of this study was to identify the optimal placement of the tibial tunnel in PCL reconstruction based on clinical studies.

METHODS

In a systematic review of the literature, MEDLINE, EMBASE, Cochrane Review, and Cochrane Central Register of Controlled Trials were screened for articles about PCL reconstruction from January 1990 to September 2011. Clinical trials comparing at least two PCL reconstruction techniques were extracted and independently analysed by each author. Only studies comparing different tibial tunnel placements in the retrospinal area were included.

RESULTS

This systematic review found no comparative clinical trial for tibial tunnel placement in PCL reconstruction. Several anatomical, radiological, and biomechanical studies have described the tibial insertion sites of the native PCL and have led to recommendations for placement of the tibial tunnel outlet in the retrospinal area. However, surgical recommendations and the results of morphological studies are often contradictory.

CONCLUSIONS

Reliable anatomical landmarks for tunnel placement are lacking. Future randomized controlled trials could compare precisely defined tibial tunnel placements in PCL reconstruction, which would require an established mapping of the retrospinal area of the tibial plateau with defined anatomical and radiological landmarks.

Anatomical relationships between Wrisberg meniscofemoral and posterior cruciate ligament's femoral insertions

Objective

To evaluate the frequency and morphometry of the Wrisberg's ligament and its relationships with the posterior cruciate ligament's femoral insertion.

Study design

Controlled laboratory study.

Methods

24 unpaired knee pieces, 12 right and 12 left were submitted to a deep dissection of the Wrisberg and posterior cruciate ligaments. The pieces were photographed with a digital camera and ruler; the Image J software was used to measure the ligaments’ insertion areas, in square millimeters.

Results

The Wrisberg ligament was present in 91.6% of the studied pieces. In those its shape was elliptical in 12 pieces (54.54%). In 68% of the knees, the WL insertion was proximal to the medial intercondilar ridge, close to the PCL posteromedial bundle. The average area for the WL was 20.46 ± 6.12 mm². This number corresponded to 23.3% of the PCL's average area.

Conclusions

WL ligament is a common structure in knees. There is a wide variation of its insertion area. Proportionally to the PCL's insertion area the WL ones suggests that it may contribute to the posterior stability of the knee joint.

Double-bundle "all-inside" posterior cruciate ligament reconstruction

The procedure for repairing the posterior cruciate ligament (PCL) has a steep learning curve and entails numerous difficult steps during surgery, because of the proximity of the neurovascular bundle, difficult passage of the graft through the posterior capsule, and risk of poor tibial fixation because of the long intramedullary trajectory of the graft. The use of instruments for retrograde reaming and a new device for adjustable cortical suspensory fixation allows for a safe, reproducible all-inside double-bundle PCL reconstruction by simplifying these difficult steps. We used anterior tibial allograft or hamstring autograft together with adjustable suspensory fixation devices that allow tensioning (after fixation) of the posterolateral bundle in extension and the anteromedial bundle in flexion.

Arthroscopically pertinent anatomy of the anterolateral and posteromedial bundles of the posterior cruciate ligament

BACKGROUND

The clock-face method to identify the femoral posterior cruciate ligament (PCL) attachment has poor accuracy and reproducibility. Measurements of clinically relevant anatomic structures would provide more useful surgical guidance. The purpose of the present study was to describe the attachments of the anterolateral and posteromedial bundles of the PCL relative to relevant landmarks to assist with arthroscopic anatomic PCL reconstructions.

METHODS

Dissections were performed on twenty nonpaired fresh-frozen cadaveric knees.

RESULTS

The distal articular cartilage margin of the intercondylar notch had a consistent shape conforming to the attachments of the anterolateral and posteromedial bundles. The mean distance (and standard deviation) between the femoral centers of the anterolateral and posteromedial bundles was 12.1 ± 1.3 mm. The distal margins of the anterolateral and posteromedial bundles were a mean of 1.5 ± 0.8 mm and 5.8 ± 1.7 mm proximal to the notch articular cartilage, respectively. On the tibia, the lateral plateau articular cartilage, the medial meniscus attachment, and an osseous ridge ("bundle ridge") separating the anterolateral and posteromedial bundles were important arthroscopic landmarks. The mean distance between the tibial centers of the anterolateral and posteromedial bundles was 8.9 ± 1.2 mm.

CONCLUSIONS

The pertinent landmarks identified during arthroscopic PCL reconstruction consistently marked the borders of the attachments of the anterolateral and posteromedial bundles. To guide femoral tunnel placement, the centers of both bundles should be triangulated relative to the reported landmarks. Furthermore, the distal edge of the femoral anterolateral bundle should be placed adjacent to the articular cartilage, whereas the posteromedial bundle should be centered, on average, 8.6 mm proximal to the cartilage margin, just distal to the medial intercondylar ridge. On the tibia, the PCL tunnel should be placed just anterosuperior to the bundle ridge, with use of the lateral articular cartilage and medial meniscus attachment to guide placement.

CLINICAL RELEVANCE

The results of the present study can assist with more anatomic tunnel placement during single and double-bundle PCL reconstructions. The results also suggest that two reconstruction tunnels are needed to reconstruct the broad femoral attachment, whereas one reconstruction tunnel should be investigated further for the compact tibial attachment.

Popliteal venotomy during posterior cruciate ligament reconstruction in the setting of a popliteal artery bypass graft

Injury to the vascular structures in the popliteal fossa during arthroscopic cruciate ligament reconstruction can be limb threatening or even life threatening. We present the first report, to our knowledge, of an isolated injury to a popliteal vein during arthroscopic posterior cruciate ligament reconstruction. Unfortunately, the venotomy led to cardiopulmonary arrest and flash pulmonary edema in this patient. Preoperative planning is paramount to assess risk of injury to vascular structures, which may be increased in patients who have had prior procedures on the affected knee. Furthermore, vascular surgery consultation preoperatively after a magnetic resonance angiogram or venogram and avoiding the use of epinephrine in the arthroscopy fluid should be considered when performing these higher-risk procedures.

An anatomic study of the posterior septum of the knee

PURPOSE

To evaluate the posterior septum of the knee and determine the presence of a safe zone that could be removed, without significant damage to blood vessels and nerves.

METHODS

Nineteen fresh unpaired adult human cadaveric knees, with no macroscopic degenerative or traumatic changes, were used in this study. Microscopic evaluation was performed by analysis of H&E, CD-34, and S-100 staining.

RESULTS

The posterior septum of the knee is rich in type II and type IV mechanoreceptors and blood vessels. The superior half has a greater number of blood vessels (21.52 ± 6.36 v 12.05 ± 4.1, P < .001), higher-caliber vessels (2.2 ± 0.89 μm v 1.41 ± 0.45 μm, P < .006), and a greater number of mechanoreceptors per field (type II, 1.8 ± 1.8 v 0.42 ± 1, P = .04; type IV, 22.6 ± 14 v 14.5 ± 9.4, P = .04) than the inferior half of the septum.

CONCLUSIONS

This study has shown that the posterior septum of the knee is highly vascularized and has a great number of type II and IV mechanoreceptors. The presence of these structures is significantly higher in the superior half of the septum.

CLINICAL RELEVANCE

If debridement of the posterior septum is necessary, it should be done at the inferior aspect so that a greater number of blood vessels and mechanoreceptors can be preserved.

Anatomy and biomechanics of the posterior cruciate ligament, medial and lateral sides of the knee

The evaluation and management of posterior cruciate ligament (PCL) injuries presents a clinical challenge to even the most experienced orthopedic surgeons. Increasing emphasis has also been placed on the diagnosis of associated ligamentous and cartilaginous injuries that may contribute to patterns of instability not solely attributed to the PCL deficiency. Although a uniformly accepted surgical technique to restore the anatomy and biomechanics of the multiligament injured knee does not exist, careful identification and management of additional ligamentous injuries are critical in achieving optimum results and avoid further insult or degradation of the knee joint owing to continued instability. Knowledge of the PCL anatomy and associated structures, combined with a clinical understanding of the biomechanics of the native tissues assist the orthopedic surgeon in treating these difficult injuries.

Knee and leg injuries

The knee plays a significant role in ambulation and the activities of daily living. During the course of these activities and its role in weight bearing, the knee is susceptible to a variety of different forces and the emergency physician should be familiar with the diagnosis and treatment of the injuries that result. In addition to following basic trauma protocols, thorough neurovascular and musculoskeletal examinations should be performed and supplemented with appropriate imaging. Emergency physicians should also consider recent developments in knee anatomy and function when evaluating the patient with an acutely injured knee.