The coronal hypomochlion: a tipping point of clinical relevance when planning valgus producing high tibial osteotomies.

AIMS: In patients undergoing medial opening wedge high tibial osteotomy (MOWHTO), soft tissue opening on the medial side of the knee is difficult to predict. When the load bearing axis is corrected beyond a certain point, the knee joint tilts open on the medial side. We therefore hypothesised that there is a tipping point and defined this as the coronal hypomochlion. PATIENTS AND METHODS: In this prospective study of 150 navigated MOWHTOs (144 consecutive patients), data were collected before surgery and at three months post-operatively. In order to calculate the hypomochlion, we compared the respective changes to the joint line convergence angle (JLCA) with the post-operative axis of the leg. The change to the medial proximal tibial angle accounts for only about 80% of the change to the femorotibial angle; 20% of the correction can therefore be attributed to non-osseous, soft-tissue changes. RESULTS: We were able to demonstrate a linear change of JLCA in a range of 0° to 5° of valgus which started when the post-operative long-leg axis was corrected beyond 2° of valgus. CONCLUSION: We found that the coronal hypomochlion occurs at 2° of valgus. TAKE HOME MESSAGE: It is recommended to plan realignment for medial open wedge high tibial osteotomy at a maximum of 2° valgus. Cite this article: Bone Joint J 2016;98-B:628-33.

The fixation strength of hydroxyapatite-coated Schanz screws and standard stainless steel Schanz screws in lower extremity lengthening : a comparison based on a new torque value index: the fixation index.

INTRODUCTION: Lengthening procedures are often complicated by loosening of pins. It has been reported that coating with hydroxyapatite improves fixation and reduces the rate of pin-track infection. MATERIALS AND METHODS: We compared 47 hydroxyapatite-coated Schanz screws (HA screws) in 12 monolateral fixators mounted at the University Hospital Hamburg-Eppendorf with 45 standard stainless steel screws in 9 monolateral fixators mounted at the St Josefs-Hospital Wiesbaden by measuring the insertion and extraction torque values. The average implantation period was 7 months for the hydroxyapatite-coated screws and 5.4 months for the uncoated screws. We established the quotient of the maximum extraction torque over insertion torque which shows the change in the fixation strength with respect to time, the fixation index. It eliminates the influence of the varying pin-bone contact. RESULTS: There was no significant difference in the rate of infection. In the Schanz screws without signs of infection the index was 1.92 for the HA screws and 0.76 for the stainless steel screws (P = 0.0002) giving evidence of the improvement of the fixation by the coating. CONCLUSION: HA coating resulted in improved fixation of Schanz screws in bone and may be useful in prolonged external fixation of the lower leg. The fixation index proved to be a simple tool for the evaluation of the fixation strength of Schanz screws.

[Lengthening osteotomies in leg length inequality]. / Verlängerungsosteomien bei Beinlängendifferenzen.

Callus distraction is the standard procedure for the operative treatment of leg length discrepancy. The deformity is analysed prior to surgery. Clinical assessment and imaging allow precise quantification. The procedure is performed by ring or unilateral frame and or i.m. nail in a continuous mode. The bone cut is performed percutaneously. Associated axial or rotational deformities are corrected simultaneously--acutely or in a continuous mode. Callus distraction is preferably applied during childhood. For good outcome control of the patient is important during the inpatient and outpatient treatment. Physiotherapy and orthotic treatment are mandatory, the same as the management of complications.

[Percutaneous epiphyseodesis. Correction of leg length inequalities and frontal plane deformities]. / Die perkutane Epiphyseodese. Behandlung von Beinlängendifferenzen und Achsfehlstellungen in der Frontalebene.

The percutaneous epiphysiodesis is recommended as a save and reliable technique. We have performed 21 distal femoral, 17 proximal tibial and 3 proximal fibular percutaneous epiphyseodeses on 25 patients. On 11 patients the epiphysiodesis was caused by an average leg length discrepancy of 2,7 cm (1-4 cm). 13 patients had angular deformity near the knee joint in frontal plane (6 valgus and 7 varus deformities). One patient suffered from a combined deformity with 3, 5 cm shortened leg and varus deviation of 10 degrees. The average deformity angle of the knee was 7,4 degrees (4 degrees -16 degrees ). 9 patients have reached maturity. On 3 patients the deformity could be corrected incompletely with residual deformity of 3 degrees, 4 degrees and 6 degrees as a direct result of late referral. On 1 patient an imminent over-correction could be avoided by hemiepiphysiodesis of the residual epiphysis. Till the present follow up the other patients have been corrected as calculated preoperatively. No complications like epiphysiolysis, infection, stiff knee or angular deformities occurred. We prefer the percutaneous epiphysiodesis as a save and minimal invasive technique instead of the open Phemister-technique to tread leg length discrepancy and angular deformities of the knee in frontal plane.

[Unilateral external fixators in the correction of deformities of the extremities]. / Deformitätenkorrektur der Extremitäten mit unilateralen Fixateur externe.

The introduction of joints in unilateral fixators enables these for multiple deformity corrections. Lengthening and shortening is achieved by changing the length of the fixator telescope. Axial corrections are performed by additive or subtractive techniques using uniplanar joints or by angulation procedures. Translation is done by direct movements of the bone fragments or by metaphyseal "double angulation". Derotation preferably is done as an acute procedure or in the "ring part" of a hybrid frame. Unilateral fixators are used to correct and temporarily stabilize until the correction is held by the intramedullary nail.

Definition, quantification, and correction of translation deformities using long leg, frontal plane radiography.

The surgical realignment of mechanical axis deviation is necessary to prevent early joint degeneration. Modern types of external fixation systems allow alignment of the mechanical axis to exact degrees. Predominately, these are corrections of angulation deformities. In some cases, the analysis of the mechanical axis deviation does not show any angulation deformity, but rather a parallel staggering of the mechanical axis lines of a bone. Such parallel staggering of the mechanical axis lines is defined as a translation deformity of the bone. In combined deformities with angulation and translation, the center of deformity can be established proximal or distal to the limit of the bone. In translation deformities, the realignment of the mechanical axis requires a parallel restaggering made by a translation-osteotomy or by a counterangulated double osteotomy. In complex deformities with angulation and translation, the translation requires separate corrective planning. In frontal plane radiographs of the standing leg, the components of angulation and translation can be established graphically or by simple trigonometric formulas. The analysis and surgical procedure to realign translation deformities or a combination of translation and angulation deformities using an unilateral fixator device are discussed.

[Correction of leg deformities. Definition, estimation and realignment of axis deviation and misalignment]. / Deformitätenkorrektur des Beins. Definition, Quantifizierung, Korrektur der Translationsfehlstellung und Durchführung von Translationsvorgaben.

The operative realignment of mechanical axis deviations (MAD) is necessary to prevent early joint degeneration and to reach normal load-bearing. Modern types of external fixation systems allow an alignment of the mechanical axis to exact degrees. Predominantly this are corrections of angular deformities. In some cases the analysis of the MAD does not show any angular deformity, but a parallel staggering of the mechanical axis lines of a bone. Such parallel staggering of the mechanical axis lines is defined as a translation deformity of the bone. In combined deformities with angulation and translation the centre of deformity can be established proximal or distal out of the limit of the bone. In translation deformities the realignment of the mechanical axis requires a parallel re-staggering made by a translation-osteotomy or by a counter-angulated double osteotomy. In complex deformities with angulation and translation, the translation requires a separate corrective planning. By using external fixator systems to perform acute or progressive corrective osteotomies the position of the Axis of Correction of Angulation (ACA) in relation to the n-CORA of the deformity has to be considered. If ACA position is not conform with the n-CORA position relevant geometric effects in relation of length and translation occur. These geometric effects to corrective osteotomies can be calculated by using simple trigonometric formulas or graphical methods. Possibilities to compensate translation and length effects are shown by using unilateral external fixator systems.