The coronal hypomochlion

Transcription

1 KNEE The coronal hypomochlion A TIPPING POINT OF CLINICAL RELEVANCE WHEN PLANNING VALGUS PRODUCING HIGH TIBIAL OSTEOTOMIES E. Heijens, P. Kornherr, C. Meister From Gelenkzentrum Rhein-Main, Wiesbaden, Germany 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 5 navigated MOWHTOs ( 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 8% of the change to the femorotibial angle; % 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 to 5 of valgus which started when the post-operative long-leg axis was corrected beyond of valgus. Conclusion We found that the coronal hypomochlion occurs at of valgus. Take home message: It is recommended to plan realignment for medial open wedge high tibial osteotomy at a maximum of valgus. E. Heijens, MD, Orthopaedic Surgeon C. Meister, MD, Orthopaedic Surgeon Gelenkzentrum Rhein-Main, Wilhelmstr.3, 593 Wiesbaden, Germany. P. Kornherr, MD, Orthopaedic Surgeon Dr. Horst Schmidt Kliniken, Ludwig-Erhardt Str., 599 Wiesbaden, Germany. Correspondence should be sent to Mr E. Heijens; heijens@gelenkzentrumrheinmain.de The British Editorial Society of Bone & Joint Surgery doi:.3/3-x.98b $. Bone Joint J ;98-B:8 33. Received 3 January ; Accepted after revision 5 January Cite this article: Bone Joint J ;98-B:8 33. Corrective osteotomies of the proximal tibia are being performed more frequently, due in part to improved implants and surgical techniques., The procedure benefits younger patients as an alternative to arthroplasty, but the indications are being extended to older patients who maintain a high level of activity. 3 Shifting the axis of the leg to the healthy or preserved lateral compartment of the knee reduces load on the damaged medial compartment and relieves pain. Hoffman et al and Lobenhoffer, Agneskirchner and Galla 5 have successfully shown how shifting the axis into the lateral compartment causes the medial compartment to open, relieving pressure on the articular cartilage with a regenerative effect. We reported changes in the joint line convergence angle (JLCA) in a pilot study of 5 patients, where the phenomenon of medial opening was confirmed. We found that the influence of soft tissue structures increases in more severe cases of osteoarthritis (OA) and influences the medial opening. No way of predicting this medial opening of the knee joint qualitatively or quantitatively has been described. We found more valgus in medial opening when the mechanical tibiofemoral angle (mtfa) was corrected beyond a certain point. We have called this point the coronal hypomochlion. Hypomochlion means the centre of rotation of a joint and the tipping effect is observed in the coronal plane. The existence of a coronal hypomochlion in medial opening wedge high tibial osteotomy (MOWHTO) surgery is the hypothesis of this study and our aim was to define its position. Patients and Methods The study involved 5 navigated MOWH- TOs which were undertaken in patients with medial compartment osteoarthritis between October 7 and November. A total of five patients underwent a bilateral procedure, with one being treated bilaterally at the same operation. The mean age of the patients 8 THE BONE & JOINT JOURNAL

2 THE CORONAL HYPOMOCHLION 9 Table I. Mean values ( ) with standard deviation (SD) for mechanical tibiofemoral angle (mtfa), medial proximal tibial angle (MPTA), joint line convergence angle (JLCA) and joint laxity FUJI groups 3 n = 8 n = 3 n = 7 n = 5 mtfa ( ) (varus).7 (SD.8) 5. (SD.3).9 (SD.7) 8. (SD.5) (valgus).7 (SD.5).8 (SD.8) 3. (SD.) 3.5 (SD 3.5) Change ( ) MPTA ( ) 8. (SD.) 85.5 (SD ) 85.5 (SD.) 83.8 (SD 3.9) 88. (SD.) 9.8 (SD 3) 93.8 (SD 3.9) 93 (SD 3.) Change ( ) JLCA ( ).3 (SD ) (SD.) 3.5 (SD.3). (SD.).9 (SD.). (SD.8). (SD.3). (SD.) Change ( ) Joint laxity ( ). (SD 3.5) 7 (SD.) 8. (SD 3.) 9.5 (SD.3). (SD.3). (SD.5).5 (SD.7) 3.5 (SD.3) Change ( )..8.. Table II. Probable final mechanical tibiofemoral angle (mtfa) for each group where the correction is the expected change in joint line convergence angle (JLCA) added to the intra-operative mtfa Target valgus in post-operative mtfa Target valgus in intra-operative mtfa Expected change in JLCA Likely final mtfa to 3 to 3 5 to to 3 to 5 was 9.5 years ( to 7). The age distribution showed five patients aged < 3 years, 3 aged between 3 and 5 years and 7 aged > 5 years. The gender distribution showed a predominance of men, 98 of 5 (5.3%). In order to determine the mtfa, a long-leg standing radiograph was taken pre-operatively and the following data were collected using the digital planning tool medi- CAD (HecTec GmbH, Landshut, Germany): mtfa, medial proximal tibial angle (MPTA), JLCA (Table I). 7- The patients were divided into four groups according to the narrowing of their medial joint space.,, comprised patients without narrowing; groups and were for intermediate stages in which the medial joint space was reduced by one third or two thirds of the outer joint space, respectively; and group 3 included all patients with no medial joint space remaining. The extent of OA determined the assignment into groups which was considered when selecting the intended correction,3 according to the protocol shown in Table II. For patients, we tried to re-align to mechanical tibiofemoral angle, for patients we tried to correct to of valgus, and for s and 3 patients we tried to correct to and 5 to respectively, as shown in Table II., The pre-operative planning and surgery were carried out by a single surgeon (EH). Intra-operatively, the mtfa, medial opening (valgus stress test) and lateral opening (varus stress test) were determined both before and after correction with the OrthoPilot HTO navigation system (Aesculap AG, Tuttlingen, Germany). Joint laxity was recorded as the difference between varus and valgus stress tests. The JLCA and changes in mtfa were also documented intra-operatively by radiographs using a raster plate. The mtfa, the MPTA, and the JLCA were measured on a long leg weight-bearing radiographs three months postoperatively. The mtfa at which the greatest change in JLCA occurred was measured from the post-operative mtfa and the change of the corresponding JLCA pre- and post-operatively. The ratio between the post-operative MPTA and mtfa was taken as an indication of the static osseous proportion of the correction which had been achieved. The remaining proportion of the correction was assumed to be the result of dynamic soft-tissue stretching. Statistical analysis. The data were tabulated and presented with standard statistical methods. The results were described using the mean and standard deviation (SD). No confirmatory tests were performed. Calculations were done using Microsoft Excel (Microsoft, Redmond, Washington). Results Varus deformity was more marked when the OA was more severe ( 3). The pre-operative JLCA values also increased with advancing OA. The MPTA values were consistent across the groups (Table I). VOL. 98-B, No. 5, MAY

3 3 E. HEIJENS, P. KORNHERR, C. MEISTER Fig. a Fig. b Radiographs showing that a) prior to surgery the joint line convergence angle (JLCA) is high when the medial compartment is closed and b) after surgery, the medial compartment has opened reducing the JLCA. ( ) 8 mtfa MPTA JLCA (º) F FI F F3 Fig. Graph showing mean change in mechanical tibiofemoral angle (mtfa) (9 ) and change in medial proximal tibial angle (MPTA) (7.3 ) for the study. The MPTA accounts for 8% of the change in mtfa. The remaining 9% can therefore be attributed to soft-tissue stretching as the medial compartment opens. (x-axis: change mtfa and change MPTA; y-axis: ( )). Fig. 3 Graph showing mean joint line convergence angle (JLCA) in each group. The joint laxity values could be described similarly to the mtfa pre-operatively; they were a mean of 3 higher in 3 than in. Following surgery, the breadth of the range between mtfa for the groups had narrowed. There was a similar trend for the JLCA and joint laxity values. Otherwise the values for the MPTA increased within the individual groups. The mean post-operative MPTA was.8 higher in 3 than in. The difference between the preand post-operative MPTAs in the individual groups represented a linear increase (Table I). The values of the medial opening (Fig. ) differed between mtfa and MPTA. The mean difference in MPTA (7.3 ) accounted for 8% of the mean difference in mtfa (9 ) (Fig. ). It was therefore assumed that 9% of the correction could be attributed to soft tissue stretching as the JLCA changed (Fig. 3). Figure 3 shows increasing change in JLCA with more severe OA and the same effect was observed in joint laxity (Fig. ). corrections ranging from to valgus were explored. From to valgus there was no joint space narrowing, the joint laxity was physiological, the ligament tension was correct with no significant changes in the JLCA. This corresponds to the ideal correction for. When post-operative valgus increased beyond a linear change in JLCA was observed (Fig. 5) such that the magnitude of changes increased with increasing valgus. This change started at valgus and increased in a linear fashion, so we were able to define the coronal hypomochlion at valgus. THE BONE & JOINT JOURNAL

4 THE CORONAL HYPOMOCHLION 3 7 Joint laxity (º) 8 JLCA (º) 5 3 Mean change in JCLA (º) F F F F3 Fig. Graph showing mean joint laxity in each group. Mean Median Valgus axis post-operatively Fig. 5 Graph showing changes in joint line convergence angle (JLCA) at specific post-operative valgus axis. Change of JLCA º F F F F Mean Linear (mean) Fig. Valgus (º) As this graph demonstrates, if an example patient from 3 has an osseous correction to valgus the additional change in joint line convergence angle (JLCA) will shift the final weight-bearing correction to the planned intended correction range 3 to 5. Discussion The main findings are the existence of a coronal hypomochlion which has an effect in corrections greater than of valgus. The intended correction was based upon the extent of OA and allocation to each of the groups shown in Table II. Having established the coronal hypomochlion, we would suggest a conservative static osseous correction of valgus in anticipation of an additional dynamic soft tissue correction which will vary in size depending on the extent of the OA (Table II, Figs and 7). In our concept of MOWHTOs, we attempted to achieve the post-operative valgus mtfa according to the group the patient was assigned to pre-operatively. Following the hypothesis of the coronal hypomochlion, the osteotomy will be positioned at a valgus mtfa of to for each group. The influence of the dynamic factor, shown by the change in JLCA, will shift the post-operative mtfa to the pre-operatively calculated mtfa outcome (Table II). Within, a dynamic effect is relevant and can be explained by residual medial instability. The additional correction ( of valgus plus X) that is desirable for patients in the higher groups is added when the operated leg is loaded due to the change in the JLCA. To our knowledge, the residual medial instability cannot be recorded with any existing tools. The previously mentioned joint laxity shows variable values. We found that an additional dynamic effect of to will affect, whereas an additional to 3 will change the static results when the leg is loaded in 3. These results indicate that the planned static correction and intra-operative change from 3 of valgus for each patient do not adequately take into account the dynamic changes in JCLA that occur under axial loading, and which can therefore lead to undesirable overcorrections. Since Fujisawa, Masuhara and Shiomi s seminal publication from 979 and further modifications by, for instance, Dugdale, Noyes and Styer 3 it is generally accepted that the post-operative axis of the leg should cross through % to 7% of the width of the tibial plateau. Noyes, Barber and Simon committed to %. With regard to the post-operative valgus, the best results are described for an anatomical valgus positioning of 8 to and a mtfa valgus positioning of 3 to VOL. 98-B, No. 5, MAY

5 3 E. HEIJENS, P. KORNHERR, C. MEISTER JLCA (º) Change of JLCA F F F F Mean Linear (Mean) Valgus (º) Fig. 7 As this graph demonstrates, if the example patient from group 3 has an osseous correction to without anticipating any soft-tissue stretching the dynamic change on weight-bearing is likely to be greater resulting in an unacceptable overcorrection. º In accordance with these observations, we recommend a post-operative valgus corridor of 3 instead, which should extend from of valgus to 5 of valgus in the mtfa. Below of valgus, there is a risk that the symptoms will not be relieved due to undercorrection. Below valgus mtfa, the coronal hypomochlion is not achieved and the transfer of the load of the medial compartment will not occur. Above 5 of valgus, the risk of overcorrection increases, which may also lead to failure. Pape, Lobenhoffer and Galla 9 describe a mathematical solution to address ligamentous laxity in symptomatic and unstable knee joints. In direct radiological comparison to the opposite, asymptomatic side, the increased width of the lateral joint space is described as an expression of ligamentous laxity and is reflected in the formula as a difference with regard to the width of the medial joint space. This formula is used to determine pre-operatively the proportion of ligament laxity in the required total angle of correction and then subtract it. It is claimed that this helps to prevent overcorrection. Pape et al 9 also describe another, graphical method. For arthritic knees with a significant difference between the medial and lateral joint space, the tibia is graphically circumscribed and the tibial joint line is then aligned with the femoral joint line so that the two run parallel to one another. The osteotomy is planned from this corrected starting position. Ultimately, the JCLA that exists pre-operatively is eliminated. This method nevertheless assumes that the JLCA is not subject to any dynamics and can be completely eliminated, or returned to zero, as a result of the re-alignment. In our experience, however, few re-alignments fully achieve this. We consider that the mathematical and graphical methods are static calculations and do not take into account the intra-operative dynamics of MOWHTO. This study is limited by the lack of available information regarding the completeness of release of the medial collateral ligament (MCL) performed at operation. However, it is now common practice to release the MCL fully and accordingly our findings are based on the assumption that this was performed in every case. A further limitation is the sample distribution with and 3 represented by few patients. Further study may be necessary to confirm if the effects observed in this study for these smaller groups are applicable to groups of larger sizes. We conclude that the static osseous components of MOWHTO directly increase the MPTA as a result of realignment. An additional dynamic component is soft tissue stretching which changes the JLCA. Both components have a direct influence on the re-alignment which is achieved surgically. The static component can be controlled with meticulous pre-operative planning and a precise operative technique. The dynamic component should be anticipated and carefully considered during the planning process. In accordance with the described observations, a 3 corridor for the post-operative mtfa is recommended. This corridor lies from to 5 of valgus. The influence of the JLCA depends on the pre-operative instability. A coronal hypomochlion has been defined as the tipping point when the medial joint line opens up beyond valgus. Therefore we recommend planning re-alignments at of valgus, whereby the change in the JLCA further corrects the end result dynamically towards valgus, depending on the severity of the OA or joint laxity. Author contributions: E. Heijens: Performed surgeries, Writing the paper. P. Kornherr: Data collection, Data analysis, Graphics, Writing the paper. C. Meister: Data collection. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. This article was primary edited by J. Scott and first proof edited by G. Scott. References. Brinkman J- M, Lobenhoffer P, Agneskirchner JD, et al. Osteotomies around the knee: patient selection, stability of fixation and bone healing in high tibial osteotomies. J Bone Joint Surg [Br] 8;9-B: Seil R, van Heerwarden R, Lobenhoffer P, Kohn D. The rapid evolution of knee osteotomies. Knee Surg Sports Traumatol Arthrosc 3;:. 3. Kohn L, Sauerschnig M, Iskansa S, et al. Age does not influence the clinical outcome after high tibial osteotomy. Knee Surg Sports Traumatol Arthrosc 3;: 5. THE BONE & JOINT JOURNAL

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