Isolated patellar tendon injuries in the pediatric population are considered rare and lack a well-accepted classification system. This injury pattern in pediatrics occurs more commonly in males around 14 years of age, typically presents with apophyseal involvement, and is frequently associated with athletic activity. The patient, in this case, was a 12-year-old male who sustained an isolated distal patellar tendon injury while running. This case is unique due to lack of apophyseal involvement commonly seen in the pediatric population. On exam, a classic high-riding patella was not seen on initial X-ray, and he was able to actively extend his knee, two clinical clues associated with the diagnostic triad of patellar tendon injury. Magnetic resonance imaging was utilized to confirm the injury pattern which prompted urgent surgical intervention to allow for initiation of rehabilitation with a goal of returning to athletic activities. The patient was able to progress to full weight-bearing at 1.5 months, achieved full range of motion at 3 months, demonstrated symmetric strength with uninvolved lower extremity at 4.5 months, and had progressed back into sporting activities by 5.75 months postoperatively. The low frequency of this injury in the pediatric population requires urgent recognition and diagnosis to allow for prompt surgical intervention followed by rehabilitation for efficient return to play.
An intact patellar tendon allows for the appropriate function of the extensor mechanism. In the pediatric population, the patellar tendon physeal structure vulnerability can lead to an apophyseal avulsion injury, and rarely, an isolated patellar tendon avulsion without apophyseal involvement can occur . Patellar tendon avulsion is most frequently reported in adolescent males (13.7–14.6 years of age) during participation in sports or other high-energy activities [1, 2]. The rare occurrence of patellar tendon avulsion in this population emphasizes the need for an accurate and timely diagnosis to prevent delays in surgical intervention . The diagnostic triad of acute onset knee pain with swelling, loss of active knee extension, and a palpable intra-patellar gap should raise suspicion for an isolated patellar tendon injury [2, 3]. There does not seem to be a consistent predisposing condition for isolated pediatric patellar tendon injuries , although Osgood-Schlatter disease, osteogenesis imperfecta, Wolfe-Parkinson-White syndrome, and attention-deficit disorder have all been implicated [1, 2]. In adults, patellar tendon rupture occurs at approximately 17.5 times an athlete’s body weight . In this case report, we discuss a distal patellar tendon avulsion repaired with suture anchor fixation technique in a 12-year-old male and his timeline for return to full activity.
A 12-year-old male student-athlete was evaluated after experiencing a “buckling” event of his knee while running. Plain radiographs and magnetic resonance imaging (MRI) were obtained by an outside provider near the patient’s secondary residence. The classic “high-riding patella” was not diagnosed on initial plain film (shown in Fig. 1), and upon clinical examination, the patient was able to actively extend the knee. This is an atypical presentation for patellar tendon injury , but MRI (shown in Fig. 2) identified a distal patellar tendon avulsion from off the tibial tuberosity with some small bony avulsion fragmentation present. Utilizing a new classification proposed by Yousef , this case would fit the criteria of a type 1B2, tendon avulsion with intact tibial tubercle. The Ogden classification is not appropriate due to a lack of bony avulsion .
Initial treatment included a hinged knee brace locked in full extension, pain management techniques including medications, compression, ice, and referral to orthopedics. Orthopedic consultation near the patient’s primary residence revealed weakness of his extensor mechanism, tenderness at the tibial tubercle, and compromised integrity of the patellar tendon confirmed on MRI. The child’s family was educated on the function of the extensor mechanism, including daily ambulation and surgical intervention was recommended. The procedure was performed under general anesthesia 12 days post-injury. An incision was made over the tibial tubercle and carried down to expose the distal portion of the patellar tendon. The patellar tendon periosteal sleeve to the tibial tubercle apophysis was found to be fully avulsed and unstable (shown in Fig. 3). This was elevated to evaluate the surface below. The undersurface of the tendon and the tibial tubercle footprint were debrided with a rongeur and curet. Small bone fragments within the patellar tendon were removed. The repair was completed using four double-loaded all suture anchors passed in a combination of mattress and side-to-side compression repair to achieve fixation (shown in Fig. 4). A trial through a full range of motion demonstrated a stable fixation without compromise under stress.
Postoperatively, the patient was placed in a hinged knee immobilizer locked at zero degrees of extension and instructed to maintain non-weight-bearing for 2 weeks. Plain films taken 2 weeks postoperatively confirmed anatomic patellar alignment (shown in Fig. 5). The patient progressed to weight-bearing as tolerated with the assistance of physical therapy but was to remain in his knee immobilizer locked in full extension for a total of 6 weeks. The patient was able to fully discharge crutches 6 weeks postoperatively.
Full range of motion was achieved at 12 weeks postoperatively and strength equivalent to the uninvolved side using isometric force plate testing at 18 weeks postoperatively. The patient completed a return to running and plyometric training program over the course of the next 5 weeks and was cleared to return to age-appropriate activities, like running, jumping, and youth football and wrestling, at 23 weeks postoperatively. A final plain film, showing proper alignment, was taken at discharge from treatment (shown in Fig. 6).
Patellar tendon avulsion injuries in the pediatric population are rare, more commonly associated with apophyseal injury at the tibial tubercle. There is not a widely accepted classification system for isolated patellar tendon injuries in the pediatric population ; however, Yousef (2017)  has proposed a classification system for knee extensor mechanism injuries, which was utilized in this case. The uniqueness of the presented case was the lack of clinical findings on initial examination including the ability to actively extend the knee and normal patellar alignment on plain films , which made utilizing the criteria proposed by Yousef  difficult. Without the identification of this injury via the MRI, care could have been delayed, leading to a poor surgical outcome due to further retraction of the patellar tendon and unsatisfactory functional outcome.
Patellar tendon avulsion has been estimated at 7 percent of knee extensor mechanism injuries in the pediatric population , with a mean age of 13.9 years , 13.7 as reported by Frey , and 14.6 as reported by Pretell-Mazzini et al. . The mechanism of injury is usually the result of direct trauma or acute knee flexion against a hardly contracting quadriceps muscle . The patient reviewed in this case study was in the lower age range of cases previously reported but was within the standard deviation identified for age by Yousef . His mechanism of injury was consistent with athletic activity; however, only 2% of injuries reported by Yousef  were a result of sagittal plane running as reported in our case. A full return to sport has been reported at 5.2 +/− 2.9 months following traumatic knee extensor injury with surgical intervention . The case reported was able to return at 5.75 months to competitive sports, including football and wrestling.
One further area of research may include the correlation of patient demographics such as body mass index and Tanner stage of development on the incidence of the different classifications of extensor mechanism injuries proposed by Yousef , such as of the quadriceps, patella, patellar tendon, tibial tubercle, or a combination of structures. The patient reported in our case report had a BMI of 34.72 kg/m , raising the question of whether body habitus may be a factor in the incidence and severity of this injury. Another limitation is the lack of established rehabilitation protocol for patellar tendon injuries in the pediatric population, which may have an impact on the timeline for return to activity. Our case used a combination of manual stretching and mobilization, gait training, open and closed chain strength exercise, and a 4-week protocol of blood flow restriction training. At the time of article submission, there is novel research showing that high-load resistance training and low-load blood flow restriction training result in similar changes in the patellar tendon, specifically patellar tendon stiffness, cross-sectional area, muscle mass, and muscle strength [9, 10]. Despite the rare incidence of this injury, prompt recognition through history and physical exam in addition to X-ray and MRI is crucial for timely surgical intervention and appropriate rehabilitation for return to play. There seems to be an increasing frequency in this injury due to more participation in childhood sports and high-energy recreational activities . Further research is needed regarding risk factors for patellar tendon avulsion, whether body habitus and Tanner stage of development have an impact on this type of injury and how use of blood flow restriction training, specifically low load, impacts healing of the patellar tendon.
Statement of Ethics
The patient’s guardian granted written informed consent for the accompanying images to be published and the case study to be written about the patient. The case study was deemed not meeting the requirements for Human Subject Research by the Advocate Aurora Health RSPP and, therefore, does not require IRB oversight. This study protocol was reviewed, and the need for approval was waived by the Advocate Aurora Health RSPP.
Conflict of Interest Statement
The authors have no conflicts of interest to declare.
The authors have no funding sources.
Stacey Hladish, Mitchell Voss, Jon Henry, and Rebecca Donnay substantially contributed to the concept of the work and the acquisition, analysis, or interpretation of the data for the work, drafted the work, revised the work critically for important intellectual content, granted final approval of the version to be published, and is in agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
Data Availability Statement
All data generated or analyzed during this study are included in this article. Further inquiries can be directed to the corresponding author.