Methods:

Fifty children (male:female, 27:23) with knee coronal plane deformities (knees:physes, 86:99), (varum:valgum, 51:35) secondary to nutritional rickets were subjected to femoral and/or tibial temporary hemiepiphysiodesis using a two-hole 8-plate. The mean age at implantation was 3.8 ± 1.5 years (range 2.5 to 5). The mean follow-up was 2.8 years (range 2 to 4). All children received a standing full-length AP radiographs of both lower limbs in neutral rotation to measure the mechanical axis deviation, tibiofemoral angle, and joint orientation angles. Tibial torsion was objectively assessed by measuring the bimalleolar axis.

Results:

The radiologic measurements, tibiofemoral angle, mechanical axis deviation, mechanical lateral distal femoral angle, medial proximal tibial angle, and Hilgenreiner-epiphyseal angle, showed a highly statistically significant improvement (P ≤ 0.001). Radiographic outcomes correlated with their clinical counterparts. The mean duration of correction of the mechanical axis was 10.8 ± 2.4 months (7 to 21). The mean follow-up for rebound of the deformity was 1.5 years (range 1 to 3).

Conclusion:

The radioclinical outcome is rewarding with a tolerable complication profile. The mechanical complications were mostly related to lengthy implant retainment encountered in severe deformities. Internal tibial torsion seems profoundly responsive to correction of coronal plane deformity. And, derotation osteotomies are rarely justified. Our proposed algorithm may be used as a decision-taking guide for achieving the desired growth modulation in a more efficient manner.

Table 1.

Surgical Technique

We approached the involved physis directly maintaining dissection extraperiosteal. We used fluoroscopy to avoid epiphyseal plate and joint line injury while maintaining parallelism of screws and their placement in midlateral plane as much as possible. A thin Kirschner wire was inserted in the central hole of the eight-plate to mark the physis. A 2.5-mm drill bit was used to insert a 3.5-mm solid screw. The epiphyseal screw was inserted first. The drill bit was advanced for only a few millimeters into epiphyseal bone, and then, the screw was tightened till it fitted into the corresponding hole. Screw length was selected so as to reach at least the midline of the epiphysis.

Outcome Measures

We conducted a full screening orthopaedic examination with observational gait analysis. We have done a goniometric measurement of the standing TFA and the transmalleolar axis in the prone position as an indicator of tibial torsion. We obtained standing full-length AP radiographs of both lower limbs in neutral rotation directly before the index surgery. The standing long radiograph was repeated once again once the limbs were clinically straightened before plate removal. The radiographic indices calculated were TFA, MAD, mechanical lateral distal femoral angle (mLDFA), medial proximal tibial angle (MPTA), and Hilgenreiner-epiphyseal angle (HEA).²⁶ The TFA was defined as the angle between the following two lines. The first line is between the center of proximal diaphysis of the femur and the center of distal diaphysis of the femur, and the second line is between the center of proximal diaphysis of the tibia and the center of distal diaphysis of the tibia. The MAD was obtained by measuring the distance between mechanical axis and midpoint of tibial epiphysis expressed as percentage to ½ of the width of tibial epiphysis. This measurement of MAD would eliminate errors arising from magnification on radiographs and augment validity of results. Deformity severity was graded into four zones by measuring the distance between mechanical axis and midpoint of tibial epiphysis expressed as percentage to 1/2 of width of tibial epiphysis (Figure1). We allowed immediate weight-bearing according to tolerance. Patients were observed at weekly intervals in early postoperative period until full functional recovery and full range of motion were achieved. Otherwise mothers were instructed to do range-of-motion exercises at home. Patients were assessed clinically at 3-monthly intervals thereafter. When the coronal plane alignment of the lower extremity was deemed clinically satisfactory, standing full-length AP radiographs of both lower limbs were obtained. The end point for plate removal was identified once the mechanical axis falls within the central one-third of the tibial plateau for both varus and valgus deformities. For both varus and valgus deformities, an accepted correction equated to a mechanical axis bisecting the medial half of the central one-third of tibial plateau and an optimal correction equated to a mechanical axis bisecting the lateral half of the central one-third. We assessed the amelioration of tibial torsion objectively by measuring the bimalleolar axis. In addition, we took history of frequent falls, child's ability to participate in normal daily activity, and more vigorous activities associated with young children. We assessed rebound clinically because of logistic challenges. We defined rebound as a deviation of >10° from the clinical TFA recorded right after implant removal.

Data Management and Analysis

The collected data were coded and tabulated using Statistical package for Social Science (SPSS 20 IBM). Descriptive statistics included the mean, SD, and range for parametric numerical data, while median and interquartile range for nonparametric numerical data. Frequency and percentage of nonnumerical data were reported. Analytical statistics included the Studentt-test was used to assess the statistical significance of the difference between two study group means. The Mann-Whitney test (U test) was used to assess the statistical significance of the difference of a nonparametric variable between two study groups. Pairedt-test was used to assess the statistical significance of the difference between two means measured twice for the same study group.

Table 2.

Table 3.

Table 4.

Surgical Indications and Outcomes

Predictors of spontaneous resolution of persistent severe lower-limb deformities despite successful medical therapy of nutritional rickets in regard to patient and disease characteristics are yet to be identified in the literature. In that regard, age and severity of deformity are crucial factors. Likewise, the exact indications of surgical interventions currently used to correct angular knee deformities in these children remain undetermined and mainly subject to surgeon's preference. Yet, a very recent prospective natural history study suggested that varum above 4 years and 18° of valgum above 9 years usually do not correct and may require surgical intervention.²⁴ The indications for surgical intervention implemented in our study are fairly similar to those proposed by the above authors in regard to genu varum.²⁴ In addition, we based our indications on clinical parameters such as symptomatic gait difficulties and deformity severity. It is noteworthy that this study²⁴ was hospital-based which makes it specifically prone to bias. In addition, the study underestimated the role of environmental and genetic predisposition related to the pathoetiology of nutritional rickets.²³ The fact that all but two tibial varus deformities with internal tibial torsion in our study have derotated in conjugation with varus correction indicates that tibial varus and internal torsion are strongly linked. The above natural history study²⁴ failed to address the fate of internal tibial torsion known to be a close associate of varus. In our study, the markedly symptomatic tibial intoeing gait was used as one of the indicators of surgery in addition to its use as an outcome measure. Because of the dynamically developing TFA in childhood,²⁷ we considered any mechanical axis traversing the central one-third of the widest diameter of the tibial epiphysis to be accepted. Yet, we opted for a more valgus orientation of the mechanical axis, ie, a mechanical axis traversing the lateral half of the central one-third. Long-term studies of guided growth around the knee suggested that varus alignment of the mechanical axis may be a precursor of rebound.²⁸ The above argues for our stratification of radiographic outcomes. The rebound phenomena has also been attributed to children with a rapid rate of correction²⁹ and to the underlying pathology.³⁰ The radiographic findings of our study demonstrated that structural realignment of the MAD after guided growth surgery around the knee can have a notable positive effect on the hip posture in terms of measurement improvement of the HEA.

Most studies on guided growth surgery were retrospective and to some extent involved heterogeneous patient and disease cohorts in addition to diverse surgical methodologies and implants.^1,10,30,31 In turn, this may undermine the capability of such studies to provide valid and generalizable conclusions. We presented the only homogeneous and prospective series on angular knee deformities in children with nutritional rickets. Various authors have reported satisfactory radioclinical outcomes in relatively small subsets of children with nutritional rickets.^1,31 This goes in line with the overall outcome of our study.

Complications and Evolved Surgical Strategy

Various studies reported screw loosening and have related it to a variety of factors namely, osteopenic bone quality such as resistant rickets, short screw length, and proximity of epiphyseal screw start point to physis.^2,9,32,33 In consequence, some authors recommended longer and or larger screws to overcome such complications.^32,33 We agree with the above authors as to the fundamental involvement of the epiphyseal screw in loosening. Yet, in our study, screw length was not an issue. The incidence of screw loosening or implant disassembly in our study was tolerable and mainly linked to degree of deformity and the overall duration of implant retainment from insertion to removal. Furthermore, this complication almost exclusively occurred in limbs where only one physis was implanted. This prompted us to upgrade our treatment protocol and attack both physes i.e., tibial and femoral—if the deformity mounted to TFA of ≥30° irrespective of the specific contribution of each physis to the overall MAD. This evolved strategy aimed at speeding the rate of correction thereby averting screw loosening. In the mild counterparts of limb deformity i.e., 20° to 30°, we achieved correction from the physis with greater contribution to deformity and neglected the other. We believe that minor distortions (10° to 15°) of the mLDFA or MPTA would be compensated as long as the mechanical axis is restored. The above authors managed this complication by a variety of methods namely screw removal, reinsertion of longer screws, conversion to an osteotomy, or simply observation.^2,32,33 In our 14 knees (16%) which developed screw loosening, only 2 knees achieved correction of the residual valgus and varus deformities through additional plating of the ipsilateral virgin physis. In the remaining 12 knees, we simply chose to observe four of them until they achieved the end point for implant removal, and the remaining two were revised by guided growth. We acknowledge that screw loosening undermines the capability of the whole implant construct to provide growth modulation and cause undue delay in correction speed. Nevertheless, this observation strategy was successful in achieving an accepted correction at the expense of lengthy implant retainment. It is noteworthy that Masquijo et al² in their series on failure of tension band plating recorded that three of nine physes had an overall time of implant retainment from insertion to removal of greater than 2 years with two implants retained for 4.5 years each. Yet, the authors failed to make any inferences from such data.² This observation lends support to our explanation for screw loosening that incriminates a long implant retainment period as a likely causation. Our surgical strategy comprised drilling the near cortex for a few millimeters and then continuing screw insertion free handed to maximize screw purchase. In addition, we aimed at crossing the midpoint of the epiphysis and metaphysis at least. Longer screws were permissible as long as the far cortex was not violated. We estimated that this screw placement strategy would safeguard against implant disassembly. The upgraded and final treatment algorithm is depicted (Figure5).

On the other hand, the incidence of screw breakage ranged from three physes (0.55%) in a series of 967 physes up to eight physes (26%) in a series of 31 physes.^1,11,34 Funk et al²¹ reported an unacceptably high rate of implant failure (58%). Yet, they defined implant failure as implant migration or breakage.²¹ The studies that reported screw breakage strictly linked it with the metaphyseal screw in Blount disease with a severe deformity and high body mass index and to some extent linked with young age.^{1,11,18,21,22,34} In our study, we encountered one screw breakage of the right proximal medial tibial plate in a young child with bilateral genu varum. A large survey study²² reported that screw breakage occurred almost always in the metaphyseal screw-not where the head meets the shank but where the shank enters the lateral cortex. This was precisely the case in our only screw breakage case. The findings of the above-mentioned authors^{1,11,18,21,22,34} and our study outcomes point out that this screw breakage complication is an inherent characteristic of Blount disease and its underlying pathoetiology. We record one limb only with rebound phenomena.

Study Limitations

We acknowledge limitations of this study. First, we relied on qualitative assessment (absent/present) of tibial torsion. We did not measure tibial torsion quantitatively per degrees of bimalleolar axis deviation with respect to the frontal plane nor did we resort to CT or MRI due to logistic challenges. It is noteworthy that the literature reports mixed results in regard to the accuracy of clinical bimalleolar axis measurement as an indicator for tibial torsion when measured against various standard reference tests.^35,36 The satisfactory outcome of the parent-reported assessment regarding frequent falls and activity of daily life provides support for our qualitative measurement of the bimalleolar axis and corroborates our conclusions in regard to tibial torsion. Likewise, we assessed the rebound phenomenon clinically only due to suboptimal resource settings. Future studies should consider using more rigorous objective clinical and radiologic tools to assess tibial derotation. We did not correlate the age of the patient with the speed of correction as our patients belonged to a homogenous age group (young children < 5 years).

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