Patellar instability is a frequent complaint of adolescent athletes with the majority (61%) of primary patellar dislocations occurring within this population 1,2.
The incidence of primary and recurrent instability is correlated with increased participation in high school athletics, which in combination with additional risk factors (sex, age, and anatomy) can increase the risk of recurrent instability up to 88% 3,4. Common fall sports, including football, soccer, and basketball, have higher risks of instability events 3. Non-operative management remains the primary treatment strategy for primary patellar dislocations; however, recent advances in understanding risk profiles are leading to surgical intervention in select cases. Patient outcomes and rate of return to sport is dependent on treatment strategy, demographic or anatomic risk factors, and chronicity of injury 3.
The purpose of this review is to highlight patellar instability with a focus on risk factors and the implication these have on guiding clinical decision making. We also pay close attention to the various treatment options, rehabilitation courses, and outcomes associated with return to play following intervention.
Mechanism of Injury and Risk Factors
There are several factors that contribute to the development of patellar instability. Participation in sports is a known risk factor for instability, as it occurs two-thirds of the time in younger, active patients under the age of 20. The average annual incidence of patellar dislocations is estimated to be 29 per 100,000 in youth ages 10-17 4. Dislocations commonly occur laterally and arise following noncontact knee twisting and valgus loading 5.
Patellofemoral joint stability is multifactorial as static and dynamic restraints contribute to overall stability 6. The medial patellofemoral ligament (MPFL) is the primary restraint to stability as the patella engages the trochlea. With MPFL insufficiency, there is up to a 49% reduction in lateral stability at zero degrees of knee flexion 6. Partial or complete tears of the MPFL are highly associated with patellar instability, in fact disruption of the MPFL associated with 85% of primary dislocations 5. Soft tissue restraints such as the MPFL and other ligamentous structures are the primary stabilizers at early degrees of knee flexion. Bony structures, such as the trochlea augment stability at higher degrees of flexion.
Abnormal trochlear morphology is an additional risk factor for patellar instability. A dysplastic trochlea cannot provide the osseous restraint needed to stabilize the patella within the trochlear groove during knee flexion 7. Dejour et. al reported this relationship as 85% of patients with a history of patellar dislocation had evidence of trochlear dysplasia 8. Patella Alta or a high riding patella is also a predisposing factor for instability that is commonly measured by the Insal-Salvatti ratio. This ratio demonstrates the relationship between patellar tendon length and patella length and is used to determine patellar height. Due to this anatomic variation a higher degree of knee flexion is required for patellar-trochlear engagement 7. This results in an increased likelihood of instability with early ranges of knee motion. Furthermore, the tibial tuberosity and trochlear groove (TT-TG) distance assesses for tibial tubercle lateralization or medialization of the trochlear groove. An increased TT-TG has also been linked to patellar instability.
Predictive models such as the Recurrent Instability of the Patella Score (RIP Score) were constructed to predict the likelihood of recurrent instability in non-operative and operative patients. This risk assessment tool considers age, skeletal maturity, trochlear morphology and tibial tubercle-trochlear groove distance to patellar length ratio (TT-TG/PL ratio) 5. Based on this model patients can be categorized as having a low, intermediate, or high risk of subsequent patellar instability events. Through the employment of this risk assessment tool, clinicians can use shared decision making to determine if nonoperative or operative management is best to decrease the incidence of recurrent instability and subsequent cartilage injury.
Management and Rehabilitation
Management of patellar instability in youth athletes is first dependent on the chronicity of the condition. Nonoperative treatment remains the standard in primary dislocations without complicating factors 9. For these first-time dislocations, early rehabilitation focusing on hip and core strength, mechanics, and stabilization in addition to gait normalization and quadricep strengthening are preferred to surgical intervention for most patient populations 10.
Rehabilitation and return to play should be dependent on first reestablishing quadriceps, gluteus, and trunk muscle stabilization followed by then gradually increasing plyometric, agility, and conditioning exercise intensity until the athlete has reestablished 90-95 percent of their quadricep symmetry. At this point, sports-specific testing and training can be introduced to continue tailored rehabilitation for return to play. If the patient reports any persistent episodes of patellar instability during these stages, progression should be halted until symptoms resolve, or surgical intervention is required 3.
Operative intervention is recommended for patients with primary patellar dislocation with unsuccessful non-operative treatment, complications, or recurrent instability. Injury complications such as large displaced osteochondral fractures or VMO avulsions, for example, require surgical correction and recurrent instability will not improve with conservative management. The risk of recurrent dislocation increases significantly after a primary event and continues to increase with each subsequent episode 9. The total re-dislocation rate following primary dislocation has been reported as high as 26%, with a marked increased risk of 52% in patients younger than 15 years 11. Risk of developing osteoarthritis also significantly increases following primary dislocation, with one study reporting that “20 percent of patients developed arthritis by 20 years after the initial dislocation” 2. Thus, definitive intervention is heavily encouraged to limit long-term complications, especially in younger patients. The nature of skeletal immaturity in younger athletes discourages the use of certain bony realignment and reconstructive procedures due to the risk of physeal disturbance 9. Thus, in this population, soft tissue stabilization procedures are preferred. Medial patellofemoral ligament (MPFL) reconstruction either on its own or in conjunction with other procedures has been found to have the most permanent and stable results 11. Following injury to the MPFL, the lateral retinaculum undergoes contraction, making lateral retinacular release with MPFL reconstruction an increasingly popular conjunctive therapy for patellar instability 12. Short-term and long-term studies have demonstrated improvements in patient-reported outcomes and recurrent dislocation rates following this stabilization 13.
Certain patients require further intervention beyond soft tissue stabilization for their recurrent instability. Patients with bony malalignment, trochlear dysplasia, and patella alta may require more involved procedures in isolation or in addition to MPFL reconstruction. These procedures may include tibial tubercle osteotomy (TTO) or trochleoplasty, and are employed based on anatomical classification 11. TTO is recommended for patients with a TT–TG distance of more than 20 mm and has been found to have positive outcomes and low failure rate when done in conjunction with MPFL as well as MPFL and lateral retinaculum lengthening. Trochlear dysplasia, however, can be a complicating risk factor in patients undergoing this intervention. For these cases of severe trochlear dysplasia, as described by the Dejour classification system, trochleoplasty is often recommended 12. Current clinical results demonstrating fair outcomes and low incidence of recurrence, however, also show a high incidence of chondral damage, postoperative stiffness, and arthritis at follow-up, discouraging the procedure except in severe and refractory cases.
Outcomes and Return to Play
MPFL reconstruction yields high return to sport rates with or without an osteotomy.. Based upon a recent systematic review and metanalysis, greater than 90% patients resume play at an average of 6.7 months after surgery. Patients also were found to have improvements in subjective reported outcomes and significantly lowered rates of recurrent instability following MPFL reconstruction 14,15.
The reliable and successful outcomes favor MPFL reconstruction and more recent techniques demonstrate decreasing failure and complication rates. Further studies done to evaluate variations in fixation technique and graft options have demonstrated no significant differences in functional outcomes over time 16. This holds true even for patients with anatomical bony abnormalities that could be contributing to their instability. Trochlear dysplasia, TT-TG distance, and patella alta have not been found to be significant risk factors for worse patient-reported outcomes. Patient who underwent isolated MPFL reconstruction for recurrent instability with some sort of bony abnormality still are reported to have good subjective outcomes, low postoperative patellar instability rate, and a high rate of return to sports 17,18.
Unfortunately, the return to sport guidelines are varied and poorly established following reconstruction. As a result, athletes run the risk of returning to play with remaining impairments that increase their chance of re-injury 3. A stabilizing brace is one such case where an athlete may feel increased confidence and comfort in their function and subsequently expedite their return to sport. However, there is little evidence to support the use of a patellar stabilizing brace after MPFL reconstruction, and the use of a brace should not be a substitute for meeting certain post-operative functional criteria for return to play 3. To avoid the risk of re-injury, athletes should progress through rehabilitation at their own pace, gradually working their way up from muscle strengthening to simple movements to finally sports-specific exercise.
The goal of patient and sport specific recovery is to encourage athletes to progress as their body allows, as confidence in their function has been described as a key obstacle in return to their previous level of performance. While isolated MPFL reconstruction has been demonstrated to have positive outcomes in return of function and play, regardless of technique, graft, or concomitant bony abnormality, patients should still be counseled on proper rehabilitation as well as the potential difficulties in reaching preinjury levels of performance 14.
Patellar instability is one of the most common injuries affecting youth athletes today, with sports participation, demographics, and anatomic variations among the most known risk factors. Various treatment options exist depending on the nature of the instability. For primary patellar subluxation or dislocation, the standard of care is typically conservative management. However, if there is recurrent instability, clinicians must further evaluate patients prior to determining definitive management. To facilitate clinical decision making in the event of recurrent patellar instability; risk stratification tools that encompass common causative factors have been developed. Use of these tools in addition to patient specific rehabilitation program can lead to high return to play rates. However, patients should be counseled on obstacles to return to pre-injury performance in the form of confidence and function. Patients with recurrent instability, thus can be offered definitive interventions that allow them to return to pre-injury activity level while greatly reducing the likelihood of a future instability event and additional poor outcomes secondary to their injury.
- Kenneth D. Weeks I, MD, Peter D. Fabricant, MD, Hannah N. Ladenhauf, MD, and, Daniel W. Green M, MS, FAAP, FACS. Surgical Options for Patellar Stabilization in the Skeletally Immature Patient. Sports Med Arthrosc Review. 2012;20(3):194-202.
- Parikh SN, Veerkamp M, Redler LH, et al. Patellar Instability in Young Athletes. Clin Sports Med. Oct 2022;41(4):627-651. doi:https://doi.org/10.1016/j.csm.2022.05.005
- Lampros RE, Tanaka MJ. Return to Play Considerations After Patellar Instability. Curr Rev Musculoskelet Med. Dec 2022;15(6):597-605. doi:https://doi.org/10.1007/s12178-022-09792-1
- Tsai CH, Hsu CJ, Hung CH, Hsu HC. Primary traumatic patellar dislocation. J Orthop Surg Res. Jun 6 2012;7:21. doi:https://doi.org/10.1186/1749-799X-7-21
- Hevesi M, Heidenreich MJ, Camp CL, et al. The Recurrent Instability of the Patella Score: A Statistically Based Model for Prediction of Long-Term Recurrence Risk After First-Time Dislocation. Arthroscopy. Feb 2019;35(2):537-543. doi:https://doi.org/10.1016/j.arthro.2018.09.017
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- Dejour H, Walch, G., Nove-Josserand, L. et al. Factors of patellar instability: An anatomic radiographic study. Knee Surg, Sports traumatol, Arthroscopy 1994;2:19–26 doi:https://doi.org/10.1007/BF01552649
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- Shimizu R, Sumen Y, Sakaridani K, Matsuura M, Adachi N. Middle-to long-term outcome after medial patellofemoral ligament reconstruction with Insall's proximal realignment for patellar instability. Asia Pac J Sports Med Arthrosc Rehabil Technol. Jul 2019;17:5-9. doi:https://doi.org/10.1016/j.asmart.2019.02.002
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