Case Report of a Traumatic Arthrotomy of the Knee Diagnosed by Ultrasound

Abstract

In this case report we describe evaluating a patient for a traumatic knee arthrotomy using ultrasound in a resource-limited medical clinic at the base of a ski area. A 23-y-old female presented with a laceration superior to the patella of the left leg. On examination, the wound tracked deep, and providers had concern for traumatic arthrotomy. Lacerations around the knee have the highest risk for traumatic arthrotomy of any joint. This risk is due to the joint capsule of the knee extending up to 12 cm proximally from the joint line of the knee, making suprapatellar lacerations a risk. Although surgical evaluation is the gold standard for diagnosing a traumatic arthrotomy, computed tomography scan has been shown to be more sensitive than the saline-load test for identifying open traumatic arthrotomies. However, computed tomography scan was not available at the ski area clinic, and the authors have found the saline-load test to be extremely painful for patients. In this case, a novel technique using ultrasound to visualize a sterile cotton swab being inserted into the wound until it contacted the knee's joint line successfully identified a traumatic arthrotomy in a proximal knee laceration. The patient was transferred to a trauma center, where she had a surgical washout of her left knee joint. In conclusion, providers should be aware of the risk of traumatic arthrotomy in wounds that are proximal/superior to the patella. In this case, an ultrasound was used to confirm that the wound entered the proximal knee joint.

Introduction

Traumatic arthrotomy is defined as a laceration occurring near a joint that disrupts the joint capsule and risks contamination of the joint. This can result in septic arthritis. ¹ Therefore, periarticular lacerations must be thoroughly evaluated for intra-articular extension given the need for emergent surgical management when a traumatic arthrotomy is discovered.

The knee joint is particularly susceptible to traumatic arthrotomy and accounts for 53 to 91% of all traumatic arthrotomies. ² The anatomic reasons for this are the superficial nature of the joint, because it lies immediately deep to the quadriceps tendon, the plane of the joint in relation to planes of movement of the knee, and the large size of the joint. The knee joint is anchored medially near the medial tibial condyle, laterally at the head of the fibula, and anteriorly at the tibial tuberosity. Superiorly, the knee joint capsule connects to the suprapatellar bursa (this combined complex is the superior joint capsule), and this complex can extend proximally up to 12 cm from the medial joint line of the knee. ³

Historically, the saline-load test (SLT) has been the most used test to evaluate for traumatic arthrotomy, but surgical evaluation remains the gold standard. ⁴ The SLT requires a large volume of fluid to be injected into the joint (155 to 194 mL in the knee) to reach 95% sensitivity for detecting a traumatic arthrotomy. ⁵ A positive SLT occurs when sterile fluid is injected into a joint and is visualized extravasating from the laceration site. The SLT can cause pain likely due to increased intra-articular pressure and distension of the joint capsule. ⁵ Recent studies have demonstrated that computed tomography (CT) performed with 2-mm-thin slices has a sensitivity between 87 and 100% for diagnosing traumatic arthrotomy.^4,6–9 A prior cadaveric study using ultrasound to identify intra-articular air after traumatic knee arthrotomy showed a sensitivity and specificity of 65 and 75%, respectively. ¹⁰ There are no guidelines from orthopedic societies defining the most appropriate testing modality to identify a traumatic arthrotomy. Therefore, clinicians must weigh their skill level with performing an SLT and their ability to control pain from an SLT with the available data on the efficacy of CT scans to inform their practice patterns.

Here we report a novel approach to evaluate traumatic knee arthrotomy by probing a periarticular knee wound with a cotton swab under direct ultrasound guidance and visualizing the cotton swab entering the knee capsule.

Case Report

Patient Information

A 23-year-old female presented to the emergency department at the base of a large ski area. Her chief complaint was a laceration to the left leg just above her patella. This occurred when her companion skied over the top of her left knee after she fell on the slope. She was able to ambulate at the scene but did not feel that she could ski and was taken down the mountain in a toboggan by ski patrol. Bleeding was controlled with a pressure dressing applied by ski patrol.

Clinical Findings

On presentation to the medical clinic, the patient had stable vital signs. It was noted that all the layers of her ski pants had been cut, and there was a 6-cm laceration superior and lateral to the patella. On examination, the patient had intact extension and flexion of the knee. There was no instability with Lachman's, posterior drawer and valgus or varus stress. She had 2+ dorsalis pedis pulses, and sensation was intact over the deep and superficial peroneal, tibial, saphenous and sural nerve distributions of the lower extremities. The wound was noted to track deep, particularly in the superomedial direction (Figure 1).

Figure 1.

Knee wound.

Table 1.

Timeline of the event.

Date	Time	Event
4/16/23	1045	Patient arrives at ski area clinic.
	1115	Initial evaluation by provider.
	1207	Point-of-care ultrasound completed.
	1244	Patient transferred by private vehicle to trauma center.
	1402	Patient arrives at trauma center.
	1658	Computed tomography results confirm traumatic arthrotomy.
	2053	Patient admitted to orthopedics service.
4/17/23		Patient to operating room for joint washout and quadriceps tendon repair.
5/3/23		Patient seen in orthopedics clinic with 70% strength in quadriceps.

Diagnostic Assessment

The differential diagnosis for this injury includes fractures of the distal femur and patella, disruption of the quadriceps tendon with resulting loss of the extensor mechanism of the knee, superficial laceration, saphenous nerve injury, and traumatic arthrotomy. The depth of the wound and the direction it tracked raised clinical concern for a traumatic arthrotomy and/or quadriceps tendon injury. The ski area clinic is resource limited and only has access to x-rays and bedside ultrasound (Mindray TE7, Mindray LLC, Mahwah, NJ) available for imaging. An ultrasound was performed of the left knee using a linear probe (Mindray L14-6N) that first identified the suprapatellar knee joint. This was accomplished by placing a sterilized ultrasound probe (using sterile gel over skin cleaned with chlorhexadine) in a cross-sectional orientation 2 to 4 cm above the patella. Using an index finger, the lateral thigh soft tissue 2 to 4 cm below the probe was palpated. This creates shear force across the joint, which appears as a sliding interface of the joint line on ultrasound. (A video created by the authors demonstrating this technique is available on YouTube at https://youtu.be/Wtk5Ok_xbyY?si=ZcNh10InmIFx5HYu.) Once the joint was located, a sterile cotton swab was inserted into the wound and gently advanced. The cotton swab was visualized moving through the soft tissue until it contacted the joint interface (Figures 2 and 3). This confirmed a traumatic arthrotomy. An x-ray was later performed that showed no fractures and demonstrated the suprapatellar soft tissue laceration with associated intra-articular gas in the patellofemoral joint space and suprapatellar recess.

Figure 2.

Annotated ultrasound image.

Figure 3.

Ultrasound image without annotation.

Follow-Up and Outcome

The patient was transferred to the nearest trauma center, where CT scan confirmed the traumatic arthrotomy, and the patient was admitted to the operating room for joint washout (Figure 4). The patient also was found to have a partial quadriceps tendon laceration, which was repaired in the operating room.

Figure 4.

Computed tomography scan demonstrating air in the knee joint.

Discussion

The key teaching point from this case is that the knee joint extends a significant distance superior to the patella. Multiple studies have explored the superior extension of the suprapatellar recess of the knee joint and have found that the joint capsule extends superiorly between 0.6 and 12 cm from the medial joint line/top of the femoral notch of the trochlea.^11–13 The extent of the variation of this superior extension of the joint indicates that emergency physicians should have a high level of concern for traumatic arthrotomy for lacerations of the anterior knee that are within 12 cm of the joint line.

To our knowledge, this is the first description of using ultrasound to evaluate the depth of a laceration and to risk stratify a patient for an open joint. Although surgical evaluation of the joint capsule is the gold standard for confirming the diagnosis of traumatic arthrotomy, CT scan has become the most used modality for evaluating for possible traumatic arthrotomy in our institution. The authors would not use this ultrasound technique to rule out a traumatic arthrotomy in a setting with access to a CT scanner given the novel nature of the procedure, the significant variance in emergency medicine providers comfort with musculoskeletal ultrasound, and the significant likelihood that it could miss a traumatic arthrotomy. Further studies are necessary to compare this ultrasound technique with CT scanning because the reliability of the technique is unknown. As in this case, it has the possibility to confirm or highly raise suspicion for a traumatic arthrotomy in moderate- to high-risk patients in a resource-limited setting. The risk of rupturing the joint capsule and converting a laceration into a traumatic arthrotomy with our novel technique is unknown but would be the equivalent of blindly probing the wound with a cotton swab, which is a common emergency department technique. The risk is likely very low with the use of a blunt object such as a cotton swab and proper ultrasound visualization techniques.

For acute anterior knee injuries, ultrasound also can be useful in identifying disruption of either the patellar tendon or the quadriceps tendon. For posterior injuries, ultrasound can identify trauma to the distal hamstring tendons, peroneal/popliteal nerves, and vascular structures. ¹⁴ There have been studies that investigate the use of ultrasound for visualizing anterior cruciate ligament tears, measuring bone translation with ligament injuries, and identifying effusions to localize an injury to a structure within the knee capsule: ligamentous injury, fracture, and meniscus injury.^15–18

Conclusion

Traumatic arthrotomy of the knee can occur when a wound is up to 12 cm superior to the joint line of the knee. Emergency medicine providers must maintain an elevated level of suspicion for these injuries. Probing a laceration under direct ultrasound visualization is a novel technique to evaluate for an open joint that can be used in a resource-limited setting to risk stratify patients. However, confirmation of traumatic arthrotomy with an SLT or a CT scan is still necessary.