Lower extremity angular alignment changes predictably under physiologic conditions during early human development. At birth, infants present with varus alignment and eventually progress to valgus alignment starting around 24 to 48 months of age. Typically, femorotibial angle stabilizes at 8 years of age.¹ Progression and increased severity of either varus or valgus angular alignment in childhood can be pathologic, resulting in abnormal gait mechanics and unfavorable functional consequences.

Varus and valgus deformities can result in altered mechanical load, and failure to treat may result in degenerative changes such as osteoarthritis.² Therapeutic strategies aimed at treating angular deformities have evolved over time, with an array of treatment options currently available such as corrective osteotomy, stapling, and percutaneous transphyseal screws. More recently, development of implant-mediated guided growth (IMGG) has spurred innovations in this technique, which is becoming an increasingly popular component of the treatment strategy for correcting lower limb angular deformities. In addition, other knee pathologies may also benefit from this treatment, such as recurrent patella dislocation, osteochondral injury, and meniscal insufficiency.

The gold standard for the treatment of severe angular deformities of the lower limb is corrective osteotomy; however, it is an invasive procedure characterized by substantial morbidity and a long recovery period. As such, the less invasive treatment option of IMGG (also referred to as mechanical hemiepiphysiodesis) may be preferred.

Unlike osteotomy, mechanical hemiepiphysiodesis allows for slow correction of angular deformities. Its use has become widespread in skeletally immature patients because subsequent bone growth after implant removal is relatively unhindered and the postoperative restrictions are much less severe. Blount and Clarke³ first described the use of staples for hemiepiphysiodesis. Epiphyseal staples act to correct angular deformities by impeding growth via compressive forces on one side of the growth plate, while allowing unhindered growth on the opposite side. The primary drawbacks to staples include physeal damage, premature closure of the growth plates, and hardware issues such as staple breakage and staple migration.

In 2007, Dr. Peter Stevens⁴ advanced the concept of IMGG further with his invention of the “8-plate,” a two-holed plate secured by two 4.5-mm screws. This dynamic construct allows the screws to pivot up to 45 degrees to accommodate physis growth⁵ and acts as a tension band whose fulcrum rests outside the physis. Staples, however, apply rigid compressive forces within the physis, which may lead to difficult removal and perhaps growth arrest.⁴ This distinction has popularized the use of the 8-plate, and multiple short-term studies indicate that the 8-plate is as effective as staple hemiepiphysiodesis for the treatment of angular deformities in children.⁵,⁶,⁷,⁸

Recently, Gottliebsen et al.⁹ conducted the first randomized clinical trial that compared the use of epiphyseal staples to the use of the 8-plate for the treatment of idiopathic genu valgum. Twenty children participated in the study, with 10 randomized to each treatment group. All 20 children achieved complete angular correction, and no statistically significant differences were found between the two groups in terms of time to correct the deformity or postoperative visual analog scale (VAS) pain scores. The main conclusion of this study was that, in agreement with previous research,⁵,⁶,⁷,⁸ there is little evidence to suggest the superiority of the 8-plate over stapling in terms of time to correct the deformity. More long-term follow-up studies that assess the incidence of rebound growth after removal of the 8-plate are necessary in order to best evaluate its long-term efficacy.

IMGG harnesses the patient’s growing bones to correct the angular deformity, either reversibly or irreversibly, depending on the technique. As such, IMGG is contraindicated in patients with a physeal bone bar,¹⁰ patients with closed physes,
or patients who are within 6 to 12 months of physeal closure (14 years bone age for females, 16 years bone age for males).¹¹ Historically, surgical treatment of lower limb angular deformities was contraindicated in children younger than the age of 8 years.¹ But more recently, children as young as 1.7 years of age are being reported,⁴ and the trend toward treating younger children is becoming more widely accepted. Table 40.1 summarizes the most recent reported ages of patients treated with IMGG for lower limb angular deformities.

A bone age determination, made by analyzing the patient’s hand radiograph, should be made in order to identify whether or not the patient has reached skeletal maturity.¹² This determination can now be made efficiently with the use of a shorthand method developed by researchers at the Hospital for Special Surgery that uses a single written criterion for each age rather than a single radiograph and multiple criteria (Fig. 40.1).¹³ For more detailed information on growth assessment in pediatric patients, see Chapter 38.

For patients with genu valgum who still have immature growth plates, IMGG is the preferred temporary corrective strategy because it permits subsequent growth upon implant removal. This technique is effective for patients who have at least 6 to 12 more months of growth remaining.⁴

When deciding to initiate IMGG therapy, it is important to consider the implant’s rate of correction in order to avoid over-or undercorrection and appropriately monitor the patient’s treatment trajectory. Ballal et al.¹² recently examined the rate at which the 8-plate corrected a cohort of 25 skeletally immature patients with either pathologic genu valgus or genu varus. The authors reported a mean correction rate of 0.7 degrees per month in the distal femur, 0.5 degrees per month in the tibia, and 1.2 degrees per month if the femur and tibia were treated simultaneously. They also found that children younger than the age of 10 years corrected at a faster rate, on average, than the children older than the age of 10 years (1.4 degrees per month vs. 0.6 degrees). Additionally, Boero et al.¹⁴ reported a mean correction rate of 0.93 degrees per month in the distal femurs of 58 patients treated with the 8-plate for either pathologic genu valgus or varus.

In a cohort of 25 patients with 47 valgus deformities treated with a tension band plate on the distal femoral physis, Guzman et al.¹⁵ reported a mean correction rate of the anatomic lateral distal femoral angle (aLDFA) of 0.96 degrees every 3 months, or 3.8 degrees per year. Consistent with the age-stratified results by Ballal et al.,¹² Guzman et al.¹⁵ found that girls younger than 11 years of age and boys younger than 13 years of age corrected at a faster rate than older children (4.5 degrees per year vs. 3.4 degrees). Table 40.2 summarizes the most recent reported
rates of correction in patients treated with IMGG for lower limb angular deformities.