Background

Coronal plane malalignment is common, with genu valgum affecting 7.1% of children and adolescents with no obvious association with age or sex. The natural course of lower extremity alignment begins in varus in infants and transitions to neutral followed by valgus alignment at approximately 8 years of age. Mature alignment is then achieved around 10 years of age. Historically, osteotomies were the gold-standard treatment for mature patients with coronal plane deformity, though these are invasive, require a period of limited-weight bearing, and have significant morbidity. For skeletally immature patients, implant-mediated guided growth (IMGG) has become a common method of treating angular coronal plane deformities. ^,

Evaluation

Malalignment (“knock knees” or “bow-leggedness”) is best assessed using standing hip-to-ankle alignment radiographs ^, While coronal plane malalignment can spontaneously resolve at younger ages, persistent deformity beyond 8-10 years of age indicates a true deformity that may benefit from IMGG. Traditionally, this represents a mechanical axis deviation (MAD) beyond the 2 central quadrants, with MAD >15 mm and >10 mm indicative of genu varum and valgum, respectively. ^, Once coronal plane malalignment has been identified, skeletal age and deformity location are critical for the determination of appropriate surgical treatment. Bilateral deformity may be treated bilaterally, while unilateral deformity may be treated unilaterally. Treatment of an asymptomatic contralateral limb may be indicated if the deformity is significant. Implant removal is performed once the deformity is corrected. Most cases of malalignment are idiopathic, but other causes include metabolic bone diseases, skeletal dysplasia, Blount’s disease, tumors, surgery, fracture, or infection. Pathologic coronal malalignment is thought to complicate or hinder the surgical treatment of a number of knee conditions such as anterior cruciate ligament (ACL) injuries, patellofemoral instability (PFI), and osteochondritis dissecans (OCD).

Indications for Implant-Mediated Guided Growth

Classic indications for IMGG include 1) at least 2 years of skeletal maturity remaining, 2) coronal deformity without limb length discrepancy >2 cm, and 3) patient and family compliance with interval monitoring. Contraindications include minimal growth remaining or the presence of a physeal bar. Implants may be placed on the distal femur and proximal tibia to correct coronal plane deformity about the knee. It is critical in the initial radiographic assessment to identify the origin of the deformity, which can arise from the femur, tibia, or both. Common radiographic measurements for knee deformities include MAD, hip-knee-ankle (HKA) angle, while mechanical lateral distal femoral angle (mLDFA) and medial proximal tibial angle (MPTA) may be used to determine the exact origin of the deformity. For example, valgus deformity about the knee originating from the proximal tibia is addressed with temporary medial hemi-epiphysiodesis of the proximal tibia to best restore an anatomic joint line. Recent studies have found that IMGG correctly improves standard alignment parameters in the vast majority of patients, with complete correction of lower knee angular deformities in up to 90% of cases. ^, In line with the known difference in physeal growth rates, the rate of correction with distal femur hemi-epiphysiodesis has been shown to be faster than the proximal tibia (0.85°/month vs 0.78°/month), though these rates are similar ^, ( Table 1 ).

The most common methods of hemi-epiphysiodesis include tension plates and percutaneous epiphysiodesis using transphyseal screws (PETS, as described by Métaizeau). While choice of intervention is largely surgeon-dependent, there are several differences that should be noted. McGinley et al. demonstrated in a sex- and age-matched cohort that PETS correct alignment at a faster rate than growth modulation plates. Other studies of PETS outcomes have raised the possibility of overcorrection with screw use due to residual growth plate damage after screw removal. ^, Similar results of overall coronal alignment correction have been observed in unmatched studies, with overall comparable complication rates compared to tension plates and staples. Regardless of the technique utilized, patients with more growth remaining have faster correction rates and demonstrate better overall correction of coronal alignment. Bone age under 12 years for girls and 14 years for boys are typically required for complete correction.

Technique for Tension Plate Placement

First the center of the physis is identified on fluoroscopy and an approximately 3 cm longitudinal incision is made. During subsequent dissection down to the physis, use of electrocautery and subperiosteal dissection is avoided to prevent iatrogenic injury to the perichondrial ring of Lacroix. When performing medial distal femoral hemi-epiphysiodesis, the vastus medialis oblique (VMO) muscle is mobilized and retracted superiorly for plate placement. The epiphyseal screw is placed following the metaphyseal screw after confirmation of plate alignment and screw placement on AP and lateral radiograph. The plate should be centered along the long axis of the bone on the lateral view to avoid iatrogenic sagittal plane deformity. It should be noted that specifically in cases of simultaneous MPFL reconstruction, often the graft sits on top of the plate. For medial proximal tibial plate placement, care must be taken to protect the medial collateral ligament (MCL).

Complications

During the corrective phase after initial implantation, routine assessments of coronal alignment are required to avoid overcorrection and to ensure appropriate timing of hardware removal. If overcorrection does occur, compensatory IMGG may be required on the contralateral side of the knee. Conversely, rebound deformity has been well-described in patients undergoing hardware removal prior to achievement of skeletal maturity, occurring in 6.7% of distal femoral hemi-epiphysiodesis cases based on a systematic review by Artioli et al. Choi et al. reviewed specific risk factors for rebound and reported that the risk increased 1.2-fold for every 1° increase in the rate of deformity correction. Partial hardware removal (“sleeper plates”) has also demonstrated complications when compared to full removal due to the possibility of bony ingrowth into the plate screw hole leading to a tethering effect and progressive deformity in the direction of correction. ^, In PETS hemi-epiphysiodesis, another complication that must be considered is rare progressive correction even after hardware removal due to potential iatrogenic physeal closure. ^, Therefore, this technique may be best utilized in patients near skeletal maturity for whom rapid growth correction is desirable, but with limited growth remaining that, limiting the potential downside of early growth arrest. Aside from over- or under-correction, potential complications relating to surgery including surgical site infection, damage to neurovascular structures, and hardware failure/symptomatic hardware.