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CORR Synthesis: Can Guided Growth for Angular Deformity Correction Be Applied to Management of Pediatric Patellofemoral Instability?

Lin, Kenneth M. MD; Fabricant, Peter D. MD, MPH

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Clinical Orthopaedics and Related Research: October 2020 - Volume 478 - Issue 10 - p 2231-2238
doi: 10.1097/CORR.0000000000001311
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In the Beginning…

Patellofemoral instability is common and debilitating, with an overall incidence of 0.69 per 1000 person-years, and it is even more common in younger and more active individuals [24, 54]. Recurrent patellar instability can cause chondral injury, osteochondral fracture, pain, disability, and arthritis.

In the 1980s and 1990s, reports of various treatment strategies to address a dislocating patella began to surface, which included nonoperative management [6, 22], isolated lateral release [46] and osteotomy [19]. In 1992, the first report of medial patellofemoral ligament (MPFL) reconstruction was published [15]; MPFL injury is now often considered the “essential soft tissue lesion” in patellofemoral instability [39]. By the turn of the century, major risk factors for patellar instability were identified in the context of surgical correction, which are still used today in clinical evaluation and risk stratification [11]. Over the last two decades, our knowledge of anatomy, biomechanics, and pathophysiology has increased. This knowledge informs a variety of evolving surgical techniques, with a focus on modifiable anatomic factors as additional therapeutic targets.

Nonetheless, complications occur in as many as 26.1% of patients undergoing soft-tissue procedures to treat patellofemoral instability, and recurrent instability is especially common in younger patients [43]. This may result in the need for secondary procedures, such as tibial tubercle osteotomy or varus-producing distal femoral osteotomy [16] to address any biomechanical malalignment such as genu valgum that may predispose to progressive laxity or recurrent instability after isolated soft-tissue procedures. Furthermore, it is our impression that the frequency of patellofemoral instability in children appears to be increasing due in part to earlier and more-structured competitive year-round sports participation along with improved diagnostic capabilities. With younger patients receiving surgery for this condition with greater frequency, it is important to minimize recurrent instability and long-term sequelae such as degenerative arthritis. Addressing predisposing anatomic factors may assist in this goal.

The Argument

In children and teens, coronal plane angular deformity is well described [20, 23, 50, 56], and plays a role in the pathogenesis of numerous intraarticular knee pathologies, including patellar instability. Unlike in adults, where osteotomy would be needed to achieve correction of angular deformity, coronal plane angular deformity correction in skeletally immature patients can be performed using less-invasive, implant-mediated guided growth techniques. Guided growth harnesses the patient’s remaining skeletal growth to correct the deformity. In the setting of angular deformity, by temporarily tethering one side of the open physis, differential growth can be achieved. Currently, the two main techniques used for implant-mediated guided growth are the percutaneous transphyseal screw [29], and tension band plating or stapling [2, 30, 49], which have been shown to effectively correct coronal plane deformity in patients who have sufficient growth remaining; these methods are associated with a low likelihood of complications [42]. Guided-growth techniques also allow deformity correction while minimizing the likelihood of complications and morbidity associated with osteotomy, namely larger incisions and substantial direct bony manipulation and necessity of osteotomy healing. Rather than performing a formal approach to create an osteotomy and then perform fixation with a large plate, a small incision is made directly over the physis and an implant such as a two-hole plate or bone staple is placed. Patients are allowed to bear weight fully, with no restrictions to immediate postoperative ROM. The implant is then removed once correction has been achieved, or it can be left in place if the patient becomes skeletally mature.

Valgus alignment is consistently associated with patellar instability both in children and adults [12, 34]. Genu valgum leads to static and dynamic biomechanical changes that predispose patients to patellar instability, including a lateralized force vector on the patella and dynamic valgus moment during gait and jumping/landing activities. Correction of genu valgum in isolation has been shown to be effective for patellar instability in adults and children [18, 36, 55]. In adults, true correction of valgus can only be achieved using varus-producing osteotomy. In children, the use of guided growth for minimally invasive correction of genu valgum has been increasingly reported and is accepted as an effective method of correcting valgus deformity [7, 8, 27, 30, 31, 37, 39, 40, 51, 57]. By applying this technique to pediatric patellar instability in the setting of genu valgum, with or without concomitant soft tissue procedures, it may become possible to effectively treat symptomatic instability and coronal plane deformity while minimizing morbidity and allowing earlier return to function.

Essential Elements

To perform a comprehensive review, we searched PubMed, Medline, and Embase for the relevant terms including guided growth, hemiepiphysiodesis, patellofemoral or patellar instability or dislocation, genu valgum, and valgus alignment. The search returned nearly 15,000 results, and we narrowed it by including only primary research articles focusing on pediatric or skeletally immature patients. Among these articles, we included only those relating to hemiepiphysiodesis or guided growth specifically in the setting of patellar instability (rather than coronal plane deformity without patellar instability).

After a thorough assessment of the search results, despite extensive studies on genu valgum and patellar instability, mostly in the skeletally mature population, we found only three relevant primary research studies [27, 37, 51]. All three studies are case series that were published within the last 5 years, reflecting the developing nature of this area of research. Although ideally we would be able to formulate treatment strategies from large prospective, randomized comparative, or controlled studies, because this is a developing field, in evaluating our current knowledge of the role of guided growth in patellofemoral instability, we will comprehensively review the three published studies on this subject. To critically appraise these three retrospective studies, we apply the Methodological Index for Non-Randomized Studies (MINORS) grading criteria [47] to each study (Table 1).

Table 1. - Studies of hemiepiphysiodesis for pediatric patellar instability graded by MINORSa criteria for noncomparative studies
Itemb Kearney and Mosca [27] Tan et al. [51] Parikh et al. [37]
1. A clearly stated aim 2 2 2
2. Inclusion of consecutive patients 2 2 2
3. Prospective collection of data 0 0 0
4. Endpoints appropriate to the aim of the study 1 1 1
5. Unbiased assessment of the study endpoint 0 0 0
6. Follow-up period appropriate to the aim of the study 1 1 1
7. Loss to follow-up less than 5% 2 2 2
8. Prospective calculation of the study size 0 0 0
Total (maximum score 16) 8 8 8
aMINORS = Methodological Index for Non-Randomized Studies.
bCriteria are according to Slim et al. [47]: items are scored 0 (not reported), 1 (reported but inadequate), or 2 (reported and adequate), with the maximum overall score being 16.

On the Science

Pathoanatomy and Pathophysiology of Patellar Instability

Although there is limited basic science evidence on this particular topic, the pathoanatomy and pathophysiology of pediatric patellar instability have been well described. Static stability of the patellofemoral joint is imparted by both soft tissue and bony structures. The major soft tissue stabilizer of the patella is the medial patellofemoral ligamentous (MPFL) complex. The MPFL is the primary restraint to lateral patellar subluxation from 0° to 30° flexion, and in biomechanical studies, it is known to provide more than 50% overall medial restraint to the patella [3]. However, bony morphology provides a substantial amount of stability to the patellofemoral joint [11], and osseous deformity can increase the risk of patellofemoral instability. In the coronal plane, the deformity that predisposes to patellar instability is genu valgum, which causes an increased quadriceps angle (“Q-angle”) between the quadriceps and the patellar tendon in the coronal plane, resulting in greater lateral force vector on the patella. Axial malalignment is critical to appreciate as well, specifically increased tibial tubercle-trochlear groove distance (TT-TG) and patellar tendon-lateral trochlear ridge distance (PT-LTR) [33]. Rotational malalignment, in the form of excessive femoral anteversion or external tibial torsion, is contributory as well. Additionally, other crucial morphological features that predispose to patellar instability include patella alta and shallow trochlear groove morphology.

Mechanistically, patellar dislocation typically occurs with a noncontact, valgus, and flexion moment at the knee, reported in up to 93% of all patients [14]. The valgus moment is a critical component of patellofemoral instability and is exacerbated by underlying genu valgum.

Predicting Rate of Correction with Guided Growth

Numerous methods for predicting the amount and rate of deformity correction have been described, mostly in the setting of limb length discrepancy rather than angular deformity correction. Nonetheless, the concepts are applicable and important to understand when planning surgical correction. Foremost, it is important to consider the patient’s skeletal age, which is typically done radiographically based on a single view of the left hand [21]. It should be noted, however, that while this technique is frequently used, it can be inaccurate in predicting final skeletal maturity in up to 50% of patients because of the historical data on which it is based during a time when skeletal maturity was reached later [5, 13]. Next, a comprehensive evaluation of the patient’s coronal plane alignment must be performed. Several important radiographic measurements (Fig. 1) can be made on a standing hip-to-ankle view, ideally using low-dose slot scanning technology [25]. Leg lengths are measured from the top of the femoral head to the medial malleolus. The mechanical axis of the limb is drawn from the center of the femoral head to the center of the ankle. In neutral alignment, the mechanical axis passes between the tibial spines, in valgus alignment it passes laterally, and in varus it passes medially. The magnitude of mechanical axis deviation can be measured and provides a quantitative sense of the extent of the deformity. Finally, the mechanical lateral distal femoral angle (mLDFA) and medial proximal tibia angle (mMPTA) can be measured to assess the contribution of each bone to the overall deformity. Both of these angles measure 87° in a mechanically neutral knee, but have normative ranges of 85° to 90° [42].

Fig. 1:
The relevant measurements for assessment of coronal plane angular deformity on standing alignment films are shown here. In this representative example, there is right sided genu valgum, with a normally aligned contralateral left lower extremity. Mechanical axis deviation (MAD) is demonstrated on the right knee, with lateral MAD measuring 12 mm. Mechanical lateral distal femoral angle (LDFA) and medial proximal tibia angle (MPTA) on the left side are within normal limits.

In males younger than 14 years and females younger than 12 years of age, rates of angular deformity correction at the distal femur and proximal tibia have been reported to be approximately 0.7° per month and 0.4° per month, respectively [2, 50]. Rates of correction in older children are slower, at approximately 0.4° per month in the femur and 0.3°/month in the tibia [50]. One multicenter study suggested that age younger than 12 years in females and 14 years in males was associated with higher likelihood of achieving full correction at both the femur and tibia [10].

In relation to radiographic measurements of patellofemoral instability, guided-growth correction of genu valgum can drastically alter TT-TG. A trigonometric model showed that when using distal femoral hemiepiphysiodesis, with every 1° of angular correction, there is a corresponding 1-mm simultaneous medial transfer of the tibial tubercle [7]. With correction using proximal tibial hemiepiphysiodesis, every 8° of correction correlates to a simultaneous 1-mm transfer of the tibial tubercle. Taken together with prior estimates (0.7° per month correction at the distal femur and 0.4° per month at the proximal tibia), with correction over the course of 1 year, 8.4-mm tubercle translation can be achieved with distal femoral hemiepiphysiodesis alone, and potentially over 10-mm translation can be achieved with additional hemiepiphysiodesis of the tibia.

It should be noted that when performing implant-mediated guided growth, a planned second operation for implant removal is routinely performed once desired correction is achieved, to avoid persistent additional overcorrection. In the case that the patient becomes skeletally mature during the correction period, implants may be retained.

What We (Think) We Know

Treatment of Pediatric Patellar Instability

Nonoperative management (including bracing, activity modification, and physical therapy) is the mainstay of treatment for patients who have sustained a first-time dislocation. For patients with recurrent or a first-time dislocation with osteochondral injury or who are at high risk of recurrence based on risk factors, surgical treatment may be recommended. In skeletally mature patients, typical procedures performed in the setting of patellar instability include MPFL reconstruction to address soft tissue laxity, tibial tubercle osteotomy to address axial alignment and patella alta, and varus-producing distal femoral osteotomy to address genu valgum, although various other osteotomies may be performed to address additional deformity (such as rotational deformity or tibial deformity). Tibial tubercle osteotomy is contraindicated in skeletally immature patients due to the presence of the open apophysis and concern for growth injury and resultant recurvatum. Several apophyseal-sparing techniques have been described, with variable results [32, 52]. Because tibial tubercle osteotomy is often avoided in the pediatric population, and genu valgum is often seen in patients with patellar instability, correction of coronal plane deformity is integral. Implant-mediated guided growth can harness the remaining skeletal growth to correct genu valgum and improve Q-angle and TT-TG without performing tibial or femoral osteotomy, as would be done in skeletally mature patients [26, 45]. Given the proximity of the femoral origin of the MPFL to the distal femoral physis [28, 35, 44, 48], and a potential medial distal femur-guided growth implant, care must be taken to minimize physeal injury or graft disruption during guided-growth implant placement (Fig. 2).

Fig. 2:
The relative position of a medial distal femur guided-growth implant to the distal femoral physis and femoral origin of the MPFL is shown here. The ideal position of a medial distal femur guided growth implant as noted on AP (A) and lateral (B) fluoroscopic views. Panel (C) shows superimposed sagittal CT images of the distal femur in a skeletally immature cadaveric specimen. As noted in the work by Shea et al. [45], the black circle represents the position of the MPFL femoral origin in patients age who are 7 years of age or older, and the white circle represents its position in patients age younger than 7 years. The superimposed solid line represents the sagittal anatomic axis of the distal femur, along which the guided-growth implant is placed, with fixation on both sides of the physis. Placement of the guided-growth implant is anterior to the femoral origin of the MPFL; however, in patients with combined implant-mediated guided growth and MPFL reconstruction, the graft will lie over the distal aspect of the plate. In the event the plate needs to be removed, special attention must be paid to avoiding graft injury when removing the implant. Conversely, the implant can be disabled by removing the metaphyseal screw only (the so-called “sleeper plate”) [26] while avoiding dissection around the graft. Panel C is adapted with permission from: Shea KG, Martinson WD, Cannamela PC, Richmond CG, Fabricant PD, Anderson AF, Polousky JD, and Ganley TJ. Variation in the Medial Patellofemoral Ligament Origin in the Skeletally Immature Knee: An Anatomic Study. Am J Sports Med. 2018;46:363-369.

Outcomes of Guided Growth in Patellar Instability

Guided growth for correction of genu valgum, whether alone or in association with other procedures such as MPFL reconstruction, has been reported to reduce symptoms of patellar instability in pediatric patients who have these operations. We found only three primary research articles studying this particular application of guided growth [27, 37, 51]. Before considering their findings, it must first be noted that these studies are lacking in several important aspects, as identified by the MINORS criteria (Table 1). All three are small retrospective series, and although all report 100% short-term follow-up, it is unclear how many patients were subsequently lost because there were no standardized follow-up protocols. Some of these patients may have developed recurrent instability that is not captured in the results, perhaps leading to inflated estimates of the apparent benefits of the procedure, and decreasing the apparent risk of complications. The only clinical outcome reported in all three studies was subjective instability, and the types of radiographic measurements reported are heterogeneous. There was no reporting of standardized knee outcome scores that quantify pain, quality of life, function, or activity. Although these studies are valuable—they comprise the entire early evidence base on this topic—their findings must be carefully considered as numerous biases are present in their study designs, in particular selection, transfer, and assessment biases, all of which may lead to overly optimistic impressions about the benefits of the treatment being studied.

In 2015, a series of 26 knees treated with isolated hemiepiphysiodesis for genu valgum in the setting of patellar instability showed substantial deformity correction and symptom improvement at a mean of 30.9 months (range 9.2 to 71.9) [27]. Radiographically, mean deformity correction of 6.8° was achieved, based on anatomic LDFA improvement. Clinically, 69% of patients achieved complete symptom resolution (no further instability events), and 31% achieved symptom reduction (less frequent subluxations or dislocations). Seventy-seven percent of patients underwent hardware removal; the remainder achieved skeletal maturity. One complication was reported: a hemiepiphysiodesis staple partially backed out, and the patient underwent reinsertion. No reoperations were reported; however, given the nonstandardized follow-up, it is possible that patients may have subsequently received procedures elsewhere.

In 2019, a similar retrospective case series of 20 knees undergoing isolated hemiepiphysiodesis for patellofemoral instability associated with genu valgum reported clinical and radiographic improvement at minimum 1-year follow-up (final follow-up range not specified) [51]. Clinically, subjective instability was the only outcome reported, with 16 of 20 patients (80%) achieving complete symptom resolution and four (20%) with recurrent instability. Notably, all four patients with recurrent instability went onto distal femoral osteotomy, a more-invasive procedure needing longer and more-involved recovery. Radiographically, mean tibiofemoral angle correction was 3.4° and mean patellar tilt angle correction was 10.3°. A subgroup analysis found that patients with recurrent instability were older, underwent smaller deformity corrections, and had more deformity before surgery. Although it cannot be concluded from these results alone, all three of these factors may be related, as more advanced age decreases the amount of growth remaining, thus decreasing the maximum amount of deformity correction that can be achieved.

A different 2019 series of eight knees receiving hemiepiphysiodesis in addition to MPFL reconstruction showed symptom resolution in seven of eight patients (87.5%) with good radiographic deformity correction at a mean of 32.8 months (range 18 to 93) [37]. Mean anatomic valgus angle improved by 9.4° and the mechanical axis improved from the lateral to the medial quadrant. Clinically, one patient, a 7-year-old male with Down syndrome, had recurrent instability. He was found to have MPFL graft pull-out, and revision MPFL reconstruction surgery was performed; he subsequently had no further instability symptoms. However, he developed a 6° varus overcorrection after implant removal, potentially due to soft tissue tethering although the true etiology is unclear (three-dimensional imaging showed no physeal bar). Additionally, one patient developed a 5° rebound valgus deformity after implant removal but remained asymptomatic. No other complications were reported. Five patients underwent implant removal; the remainder achieved skeletal maturity.

The current data, which are of limited quality, suggest that isolated hemiepiphysiodesis and hemiepiphysiodesis with MPFL reconstruction leads to good radiographic deformity correction and short-term subjective resolution of instability symptoms in 66% to 87.5% of patients. Most patients undergo planned implant removal to stop the deformity correction if proper correction is achieved before skeletal maturity. Based on these limited studies, surgical morbidity appears to be low, with few complications in studies with heterogenous short- to mid-term follow-up and immediate full weightbearing and ROM postoperatively. A portion of patients acquire a rebound deformity, although the true frequency of this is unknown given the heterogeneous, nonstandardized follow-up.

Knowledge Gaps and Unsupported Practices

Although guided growth is a reliable and widely used technique for isolated deformity correction, it is potentially also an important adjunctive tool for management and prevention of patellar instability associated with genu valgum in the pediatric population. Nonetheless, there are still important gaps in our knowledge. First, better clinical evidence is needed to understand the true results of this operation clinically and radiographically, both in the short- and long-term. It is likely that further research will help identify complications or shortcomings of the procedure, which will help direct future research as we aim to improve our techniques and outcomes for treating pediatric patellar instability. As noted, the evidence now is limited overall both in its amount and its quality. Beyond that, there are several specific areas that require further study, as we will outline here.

Anatomic Structures at Risk

During routine tension band plating for temporary hemiepiphysiodesis for correction of a genu valgum deformity, the medial aspect of the medial femoral condyle and epicondyle must be dissected extra-periosteally. Important medial-sided structures include the MPFL, MCL, perichondral ring of LaCroix, and joint capsule. In cadaveric studies, it has been shown that standard medial tension band plate placement leads to partial dissection or perforation of the MPFL in 50% of cases, perforation of the medial capsule and inadvertent intraarticular exposure in 50%, and no impingement or damage to the MCL [1].

Rebound Phenomenon

Rebound phenomenon is the possibility for partial recurrence of coronal plane angular deformity after tension band plate removal in patients who still have growth remaining and “grow back” toward their deformity [17]. As guided growth becomes more widely used, rebound growth after hemiepiphysiodesis is becoming increasingly recognized. Although little is known about its cause and physiological etiology, it has been reported to occur in up to 79% of patients after hemiepiphyseal stapling [38]. Rate of correction, BMI, age, and magnitude of initial deformity have been correlated with increased risk of rebound growth [38]. Controlled animal studies have shown that rebound growth does not occur immediately but rather begins several weeks after removal of the guided-growth implant [9]. In the setting of coronal plane angular deformity of the knee, rebound deformity of up to 10° has been reported [27]; however, in the limited evidence on hemiepiphysiodesis in patellar instability, there are no reports to our knowledge correlating rebound deformity with recurrence, although this is certainly a concern. Regardless, there is no current consensus on whether overcorrection is required to account for rebound growth.

Contralateral Guided Growth

It is known that around 10% of pediatric patients with patellar instability will develop contralateral symptoms [41]. Within the overall population of children with patellofemoral instability, it is unknown what percentage of patients who have genu valgum will go on to develop contralateral symptoms. The proportion is likely higher than those without genu valgum, as valgus alignment is a known risk factor for patellar instability. Because implant-mediated guided growth is a minimally invasive procedure that allows patients immediate full mobility in the operative extremity, contralateral guided growth in the setting of bilateral genu valgum but one-sided patellar instability may be a consideration for maintaining cosmetic symmetry and preventing contralateral symptoms.

Barriers and How to Overcome Them

Many of the current barriers are related to volume and quality of evidence and general awareness of this specific application of guided growth. Currently, there are only three case series reported on hemiepiphysiodesis for correction of genu valgum in the setting of pediatric patellar instability. However, as treatment of pediatric patellar instability becomes more common, use of guided growth is increasing. As clinical experience increases, longer-term outcomes data will become available. It is important for future studies to focus on mitigating selection, transfer, and assessment bias, which all have the potential to make the study intervention appear to have better outcomes than it may actually have in reality. A more nuanced understanding through higher-quality research may eventually help stratify patients into risk categories that will guide indications for genu valgum correction as a component of patellofemoral instability surgery. This might include prospective multicenter study of skeletally immature patients with patellofemoral instability, such as is being done by the newly-formed Justifying Patellar Instability Treatment by Early Results (JUPITER) study group [4]. As awareness, enthusiasm, and clinical experience with this technique increase, we hope that a greater research interest is cultivated, allowing for these types of studies.

5-year Forecast

We speculate that in the near future there will be a growing body of evidence to further evaluate the efficacy of guided growth in patellofemoral instability, as this is a field that has only recently emerged. As higher-quality evidence comes from comparative, prospective studies with good follow-up and reliable outcome measures, our understanding of surgical results, patient satisfaction, indications, and surgical technique will improve. With higher quality data, in the future we may eventually have the clinical capability to risk-stratify young patients with patellofemoral instability based on severity of coronal malalignment, age and skeletal maturity, injury pattern, and other patient factors that will direct surgical indications for guided growth an also provide targets for correction. Furthermore, we believe that through basic science and biomechanical studies, we will better understand the biological importance of bony alignment in soft tissue healing, by applying the principles of stress, strain, and other forces to tissue healing and joint homeostasis. Finally, if further high-quality evidence can continue to shed positive light on this technique, we postulate that this concept may become increasingly used for other intraarticular knee conditions, including ACL reconstruction, tibial eminence fracture repair, discoid meniscus repair, and osteochondritis dissecans repair.


We thank Madison R. Heath BS for her assistance with manuscript and figure preparation.


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