Surgical accuracy in high tibial osteotomy: coronal equivalence of computer navigation and gap measurement

Abstract

Purpose

Medial opening wedge high tibial osteotomy (MOW HTO) is now a successful operation with a range of indications, requiring an individualised approach to the choice of intended correction. This manuscript introduces the concept of surgical accuracy as the absolute deviation of the achieved correction from the intended correction, where small values represent greater accuracy. Surgical accuracy is compared in a randomised controlled trial (RCT) between gap measurement and computer navigation groups.

Methods

This was a prospective RCT conducted over 3 years of 120 consecutive patients with varus malalignment and medial compartment osteoarthritis, who underwent MOW HTO. All procedures were planned with digital software. Patients were randomly assigned into gap measurement or computer navigation groups. Coronal plane alignment was judged using the mechanical tibiofemoral angle (mTFA), before and after surgery. Absolute (positive) values were calculated for surgical accuracy in each individual case.

Results

There was no significant difference in the mean intended correction between groups. The achieved mTFA revealed a small under-correction in both groups. This was attributed to a failure to account for saw blade thickness (gap measurement) and over-compensation for weight bearing (computer navigation). Surgical accuracy was 1.7° ± 1.2° (gap measurement) compared to 2.1° ± 1.4° (computer navigation) without statistical significance. The difference in tibial slope increases of 2.7° ± 3.9° (gap measurement) and 2.1° ± 3.9° (computer navigation) had statistical significance (P < 0.001) but magnitude (0.6°) without clinical relevance.

Conclusion

Surgical accuracy as described here is a new way to judge achieved alignment following knee osteotomy for individual cases. This work is clinically relevant because coronal surgical accuracy was not superior in either group. Therefore, the increased expense and surgical time associated with navigated MOW HTO is not supported, because meticulously conducted gap measurement yields equivalent surgical accuracy.

Level of evidence

I.

Knee Surg Sports Traumatol Arthrosc (2016) 24:34103417 DOI 10.1007/s00167-016-3983-7

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Surgical accuracy in high tibial osteotomy: coronal equivalence of computer navigation and gap measurement

S. Schrter1 C. Ihle1 D. W. Elson2 S. Dbele1 U. Stckle1 A. Ateschrang1

Abstract

Purpose Medial opening wedge high tibial osteotomy (MOW HTO) is now a successful operation with a range of indications, requiring an individualised approach to the choice of intended correction. This manuscript introduces the concept of surgical accuracy as the absolute deviation of the achieved correction from the intended correction, where small values represent greater accuracy. Surgical accuracy is compared in a randomised controlled trial (RCT) between gap measurement and computer navigation groups.

Methods This was a prospective RCT conducted over 3 years of 120 consecutive patients with varus malalignment and medial compartment osteoarthritis, who underwent MOW HTO. All procedures were planned with digital software. Patients were randomly assigned into gap measurement or computer navigation groups. Coronal plane alignment was judged using the mechanical tibiofemoral angle (mTFA), before and after surgery. Absolute (positive) values were calculated for surgical accuracy in each individual case.

Received: 23 August 2015 / Accepted: 5 January 2016 / Published online: 22 January 2016 European Society of Sports Traumatology, Knee Surgery, Arthroscopy (ESSKA) 2016

Results There was no signicant difference in the mean intended correction between groups. The achieved mTFA revealed a small under-correction in both groups. This was attributed to a failure to account for saw blade thickness (gap measurement) and over-compensation for weight bearing (computer navigation). Surgical accuracy was 1.7 1.2 (gap measurement) compared to 2.1 1.4

(computer navigation) without statistical signicance. The difference in tibial slope increases of 2.7 3.9 (gap

measurement) and 2.1 3.9 (computer navigation) had

statistical signicance (P < 0.001) but magnitude (0.6) without clinical relevance.

Conclusion Surgical accuracy as described here is a new way to judge achieved alignment following knee osteotomy for individual cases. This work is clinically relevant because coronal surgical accuracy was not superior in either group. Therefore, the increased expense and surgical time associated with navigated MOW HTO is not supported, because meticulously conducted gap measurement yields equivalent surgical accuracy.

Level of evidence I.

Keywords Computer navigation Gap measurement

Surgical accuracy High tibial osteotomy Knee osteotomy Medial compartment osteoarthritis

Introduction

Coronal plane deformities overload the affected knee compartment [1] resulting in osteoarthritis progression [6, 38, 41]. Medial opening wedge high tibial osteotomy (MOW HTO) shifts load towards the lateral compartment [2], improving symptoms and postponing or avoiding arthroplasty. In the modern latter years [39], the selection of

The study was performed with approval from the ethical committee at the University of Tbingen (142/2008MPG2). This is trial number DRKS00005614, included in the German register for clinical trials (approved primary register of the WHO network).

* D. W. Elson [email protected]

1 Department of Traumatology and Reconstructive Surgery, BG Traumacenter Tbingen, University of Tbingen, Schnarrenbergstr. 95, Tbingen, Germany

2 Department of Orthopaedics, Queen Elizabeth Hospital, Gateshead, UK

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intended correction has been tailored to each patients individual problem [13], which may be inuenced by surgical indication [13] and the degree of deformity or arthritis [29, 30]. So a set target point (or range) for all patients, is inconsistent with this bespoke approach. Accordingly, the judgement of achieved correction must also evolve to reect this individualised approach. We introduce the concept of surgical accuracy, dened as the absolute deviation of the achieved correction from the intended correction and reported for each patient. Inevitably the important question is how can we improve surgical accuracy. Four broad techniques are described below: patient-specic instrumentation, uoroscopic conrmation, gap measurement and computer navigation.

In arthroplasty, patient-specic instrumentation [12] has demonstrated improved accuracy over manual alignment [31] but not computer navigation [32]. Its application to high tibial osteotomy remains experimental [23]. Fluoroscopic conrmation, described by Dugdale and Noyes [7], simulates the weight-bearing axis with radiopaque objects (taught cables, alignment rods and raster plates) relative to the supine lower limb. There is considerable room for rotational error and parallax, with no good evidence of effectiveness. In one study a uoroscopic conrmation group only achieved the coronal target in 23 % of cases [20].

Gap measurement capitalises on high reliability with digital planning software [37] or radiographic annotation [11, 26] to reproduce the planned opening distance at the medial tibial cortex. The osteotomy gap is surgically controlled and then measured with varying precision. In twogap measurement series, 8385 % of cases reported surgical accuracy within 15 % of the intended correction [10, 42].

Computer navigation measures angles for the whole limb, and high precision has been demonstrated in arthroplasty [5, 19] and HTO [3, 14, 16, 20, 27, 34, 35]. However, closer scrutiny reveals methodological deciencies such as historical control groups [34], undened intended corrections [27, 34], variable chronology of radiological follow-up [16, 27] and small sample sizes [20]. Navigated surgery is time consuming and expensive [27], so justication should be sought from improved surgical accuracy.

This randomised controlled trial (RCT) compares gap measurement and computer navigation techniques in MOW HTO, with the null hypothesis that neither group will generate higher coronal surgical accuracy.

Materials and methods

This trial recruited 120 consecutive patients undergoing MOW HTO where closed envelopes were chosen to randomise patients to either group. There was no crossover

between groups, with data analysed on an intention to treat basis. Informed consent was obtained from all individuals participating in the study. Inclusion criteria were patients aged over 18, with symptomatic varus malalignment and medial compartment osteoarthritis, or active patients with known articular cartilage lesions. The trial protocol excluded patients with previous or acute infections of the knee joint or subsequent surgeries with a confounding inuence upon alignment. Therefore, seven patients (6 %) were excluded: two revised implant failures, two revised over-corrected, one revised under-corrected case, one delayed union requiring bone grafting with plate exchange and one deep infection treated with early implant removal and external xation to maintain the gap. Of the seven excluded patients, three were from the gap measurement and four from the navigated group.

Preoperative planning and surgical technique

Pre-operative deformity analysis was conducted with medi-CAD digital planning software (Hectec, Landshut, Germany). The Cartesian coordinates of anatomical landmarks determine the coronal plane mechanical and anatomical axes to generate conventional alignment angles [33] to one decimal place (Table 1; Fig. 1). There is high test retest reliability with intra-class correlation coefcients between 0.896 and 0.995 [37]. Each patient had a bespoke intended correction selected based on their indication, anatomy and pathology [13]. Typically, this was a correction aimed between 2.0 and 3.0 valgus [14], for patients with moderate varus osteoarthritis. This intended correction was simulated with mediCAD to generate both a correction angle and opening distance, to one decimal place. The osteotomy technique was similar to previous descriptions [28, 40]. Two K-wires entering the tibia at the metaphyseal are, parallel to the tibial slope, were guided towards the tibiobular joint tip with uoroscopy. The transverse plane osteotomy was made to within 1.0 cm of the lateral cortex. The biplanar limb of the osteotomy was created proximally and under the tibial tuberosity as visualised at an angle of 110.0. The osteotomy was completed, osteotomes were stacked, and the gap was gently spread. Bone graft was not used in the osteotomy gaps, which were xed using the TomoFix plate (DePuy Synthes, Solothurn, Switzerland) in the standard manner [28, 40].

Table 1 Conventional abbreviations for alignment parameters [33]

Abbreviation Alignment angle

mTFA Mechanical tibiofemoral angle MPTA Medical proximal tibial angle

JLCA Joint line convergence angle

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Fig. 1 Typical digital deformity analysis and digital planning output using mediCAD software (Hectec, Landshut, Germany)

Gap measurement group

In the gap measurement group, the osteotomy was held open to the planned opening distance with a posteriorly placed laminar spreader. This distance was conrmed using corpectomy callipers (able to record variation of 0.1 mm) before plate xation with no further conrmation of alignment.

Computer navigated group

The OrthoPilot (BBraun, Tuttlingen, Germany) system consists of a computer workstation attached to infrared cameras and a passive optical sensor. Arrays with disposable optical markers were rigidly screwed to both tibia and femur to locate the limb segments. The cameras were positioned such that the reected infrared beams were detectable in both knee exion and extension. The hip, knee and ankle centres were registered [21, 22] allowing calculation of the mechanical tibiofemoral angle (mTFA), to one decimal place. Pre-correction alignment measurements were recorded from the OrthoPilot screen. Once the osteotomy gap was established, this was opened to create the intended

mTFA under navigated control. Weight bearing does not occur through the supine limb, and to compensate for the effect of standing, an on screen mTFA was deliberately selected to be 1.0 less than the mediCAD plan. The Tomo-Fix plate was applied and xed, the nal alignment was measured, and the screenshot was saved.

Followup assessments

Radiographic assessment included orthogonal knee views and weight-bearing anteroposterior long-leg views taken before surgery and 6 weeks after surgery. The same observer (SS) used mediCAD planning software (Hectec, Landshut, Germany) to record conventional alignment parameters [33] from each radiograph (Table 1). Tibial slope was measured on lateral radiographs relative to the proximal tibial axis [4].

Analysis of surgical accuracy

Subtracting the achieved correction from the intended correction generates either an under-correction () or over-correction (+). For this analysis, the results were mathematically

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converted to absolute values (positive integers) by taking the square root of the squared value. Thus surgical accuracy was reported as deviation from the intended correction, with directionless magnitude and lower values representing greater accuracy. Magnitudes >4.0 were considered to be inaccurate and subsequently revised after recognition.

Ethics

The study was part of the Tbingen Osteotomy Trial, which received ethical approval from Tbingen University (142/2008MPG2) and was registered (German clinical trials register: DRKS00005614).

Statistical analysis

SPSS version 21 (IBM, Chicago, Illinois) and GraphPad Prism version 5 (GraphPad Software, La Jolla, CA) were used for statistical analysis and graphical representation. Mean comparison was performed using dependent or independent t tests. One way analysis of variance performed

with repeated measures was used, with subsequent pairwise comparisons analysed by alpha-adjusted Bonferroni corrections, where P < 0.05 was considered signicant. A power calculation indicated that a sample size of 60 could detect a difference in mTFA of 1.0 with signicance of P < 0.025 and power at 80 %.

Results

There was no signicant difference between demographics or pre-operative deformity for patients in either group (Table 2). The pre-operative plans generated almost identical mean opening distance, without statistical difference. Table 3 demonstrates that navigated surgery took 1318 min longer to perform.

When compared between groups, the intended correction was almost identical, as was the achieved correction, without a statistically signicant difference (Table 4). Both groups were under-corrected, by 0.8 2.0 (gap meas

urement) and by 0.9 2.0 (computer navigation). Sur

gical accuracy was 1.7 1.2 (gap measurement) and

2.1 1.4 (computer navigation) (Fig. 2). This compari

son between groups was not statistically signicant. In the sagittal plane, the tibial slope increased by 2.7 3.9 (gap

measurement) and 2.1 3.9 (navigated) which was a sta

tistically signicant difference (P < 0.001) but not a clinically relevant difference (0.6).

Discussion

The most important nding of this study was equivalence in coronal surgical accuracy between groups; hence, the operation time in navigated surgery was extended without justication. This remains a controversial area where some

Table 2 Patients characteristics

Variables Gap measurement group

Computer navigation group

P value

Patients 57 56 n.s. Age (years) 47 8 45 8 n.s.

Gender (M/F) 36/21 47/9 n.s. Body mass index

(kg/m2)

28.3 4.8 29.1 4.5 n.s.

Side (R/L) 37/20 26/30 n.s. mTFA () 5.4 2.5 5.3 2.5 n.s.

MTPA () 86.0 3.0 86.0 3.0 n.s.

JLCA () 2.3 1.6 2.5 1.7 n.s.

Table 3 Surgical time Surgical time (min) Gap measurement group Computer navigation group P value

With arthroscopy 84 17 97 25 0.016

Without arthroscopy 66 17 84 17 0.015

Table 4 Alignment parameters with values reported as means with standard deviations

Surgical accuracy is generated from absolute values, such that deviation from the intended correction is independent of direction

Measured angle () Gap measurement group Computer navigation group P value

Intended correction 2.5 0.8 2.7 0.6 n.s.

Achieved correction 1.7 2.2 1.8 2.1 n.s.

Surgical accuracy 1.7 1.2 2.1 1.4 n.s.

Tibial slope increases 2.7 3.9 2.1 3.9 P < 0.001

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Fig. 2 Boxplot distribution for surgical accuracy (absolute values for deviation of achieved correction from intended correction) between the two groups, using the parameter mTFA (). There was no statistically signicant difference between groups

studies report no difference [35], and others have demonstrated greater accuracy with computer navigation [20] albeit compared to an inaccurate uoroscopic conrmation group. Therefore, it could be inferred from this study that gap measurement must be more accurate than uoroscopic control, to reach equivalence with computer navigation, but this hypothesis was not specically tested. Other controlled studies found that navigation reduced outliers [34] and under-correction [3] or improved precision [2, 24]. Proponents of navigation [14, 16, 27] in studies without controls, describe it as a useful tool allowing intra-operative control of alignment.

Although the surgical accuracy found in this paper was within an acceptable tolerance (mTFA < 2.1 1.4),

both groups were under-corrected at the rst radiographic assessment, with potential reasons discussed below. Digital planning [37] and radiographic annotation [11, 26] have high reliability. Gap measurement surgically creates this opening distance according to the reliable pre-operative plan. Variations in the gap sizes are to be expected between individuals, such that opening all gaps to a standard size (1 cm) or eyeballing is practice, which should be discouraged. Precision in measurement is mandatory, with appropriate devices including a pre-cut paper ruler, the gap-measuring device or corpectomy callipers. A paper ruler can be cut to within half a millimetre, mounted on a small artery clip and held against the gap. The osteotomy gap-measuring device (Synthes, Solothurn, Switzerland) is a metal wedge wide enough to support the medial cortices when tapped into the gap, and the outer sleeve is advanced against bone, to read within half a millimetre. Alternatively, the arms of the corpectomy callipers (Synthes, Solothurn,

Switzerland) can be opened inside the gap and xed in position before withdrawal, to give a reading within 0.1 of a millimetre. Lag (golden) screw application generates compression forces across osteotomies, which are increased with steeper screws or with further plate bending at terminal screw advancement. Whilst the majority of force is concentrated on the lateral cortex, in high compression this propagates across the osteotomy, which reduces the medial gap. Anecdotally, surgeons anticipate 0.51.0 mm of gap loss and compensate by opening wider before the lag screw. Another important consideration is saw blade thickness, which must be added to the planned opening distance because bone is lost under the saw cut. During manuscript preparation, it was recognised that saw blade thickness had been omitted from calculations; in retrospect, this is likely to account for the under-correction observed in the gap measurement group.

The OrthoPilot navigation system is reliable when the leg is extended [15], but caution has been advised with soft tissue tension when performing navigated HTO in supine patients [25]. So in the navigated group, 1.0 was deliberately reduced from the planned mTFA to compensate for weight bearing, with the nal navigation output 0.8 2.0

less than the plan. The assumption that weight bearing inuences the achieved correction by 12 has been stated elsewhere [20, 25]. At the rst weight-bearing radiograph, the navigated group was under-corrected by 0.9 2.0.

So this study has demonstrated minimal effect of weight bearing on coronal alignment compared to supine recordings. In retrospect 1.0 was an over-compensation, which resulted in the under-correction for the navigated group. Perhaps improved surgical accuracy could have been achieved by directly following the navigation without this manual adjustment, but study repetition would be required to conrm. Surgeons can simulate weight bearing with an axial force [25], but this study emphasises the need for a controlled study to determine the correct method [25].

Alignment and corrections are quantied as angles (mTFA [14, 37] or hip knee ankle angle [17]) or percentages (Mikulicz point, where the weight-bearing axis transects the tibial width [11, 13, 34]). Whilst both are equally valid representations of alignment, this study uses the mTFA. As a relevant example, our own working group have previously reported a knee osteotomy group, under-corrected with mTFA of 2.5 1.4 [36] where alignment parameters

were presented as true means. However, there is every possibility that the magnitude of over-corrections could mirror that of under-corrections, resulting in a mean close to zero, falsely implying high accuracy. This manuscript uses an improved mathematical representation with absolute values of individual deviation from the intended correction. In the example [36] true values yielded a standard deviation of

1.4, but absolute values produced surgical accuracy with a

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standard deviation of 3.4. Evidently, the method of mathe

matical representation yields different surgical accuracy; the units, however, are interchangeable (angles or percentages). A set of such absolute values can be described simply with a distribution and calculated mean. More sophisticated statistical representations include the coefcient of variance [35] or outlier rates around a dened tolerance [14, 34]. Ribeiro et al [35] demonstrated a higher variance for the control group (97 %), compared to their navigated group (58 %), a nding not emulated here, with no difference between groups and low standard deviation. The 3.0 tolerance set by Gebhard et al [14] was achieved in 85 % of their recruited patients. In this study, the same 3.0 tolerance threshold would be achieved by 92 % of the gap measurement group and 89 of the computer navigation group. A contrasting study [3] found less outliers in the navigated group. However, a tolerance of 3.0 may be unacceptably high, because in this protocol, deviation exceeding 4.0 resulted in revision. Alternatively a low tolerance of 1.5 yielded 73 % as outliers [36]. Perhaps an acceptable tolerance threshold lies somewhere between 1.5 and 3.0. The newly emerging osteotomy registries [9] aim to relate surgical accuracy to patient outcomes to clarify acceptable tolerance based on more than just alignment. The dened tolerance approach is a pragmatic acceptance of the various factors, which inuence achieved correction, but remain difcult to control.

Typically, the tibial slope increases after opening wedges and decreases after closing wedges [8]. The slope increase was greater in this gap measurement group (P < 0.001) despite the lack of surgical attention to the sagittal plane in the navigated protocol. Whilst this 0.6 difference was statistically signicant, it was not clinically relevant. Some authors [3, 35] have found that navigated surgery reduces the magnitude of tibial slope changes, whilst others have not [24]. In Ribeiro et als control group [35], the slope increased by 4.8 with a 2.0 increase in the navigated group. This studys navigated group demonstrated the same slope increase (2.1). The gap measurement slope increase of 2.7 is closer to the navigated group than the control group from Ribeiro et al. [35]. Iorio et al [20] present favourable accuracy in a small, navigated group when compared to uoroscopic conrmation. The study has small sample sizes without power calculation. In the authors opinion, tibial slope increase is dependent on surgical technique with preventative measures described [18]. The spreader should be positioned posteriorly to allow the osteotomy to open as a trapezoidal gap, which is narrower anteriorly. Indeed both anterior and posterior osteotomy gap size recordings have been included in registry datasets [9] to offer future insight. So, whilst navigation may offer slightly better control of tibial slope, meticulous attention to surgical technique with gap measurement can yield similar results to previously published navigated data.

This study has several limitations. A minority (10 %) of patients were unable to fully extend at the 6-week long-leg radiograph, which could lead to inaccurate measurement of alignment parameters [14]. An independent observer of the radiographic outcomes could have strengthened the study design. Systematic biases were inadvertently introduced into both groups, which became apparent during manuscript preparation. The gap measurement procedures did not account for the saw blade thickness, and the navigated procedures were over adjusted for weight bearing. Both of these systematic biases caused an under-correction, which appears to be of a similar magnitude. It is therefore speculative whether a difference between gap measurement and navigated groups would have been observed if these systematic under-corrections had not occurred in this study. Considered another way, the surgical accuracy of both groups could be improved with the lessons learned during the conduct of this study, which has clinical relevance for osteotomists who use either technique. Namely saw blade thickness must be added during gap measurement and compensatory adjustment for weight bearing should be avoided when using computer navigation. The concept of surgical accuracy for individual knee osteotomies is clinically relevant to osteotomists who perform either gap measurement or computer navigation, where this paper demonstrates equivalence.

Conclusion

This study showed equivalent surgical accuracy between gap measurement and computer navigation in MOW HTO. Both techniques are accurate. The conduct of this study has demonstrated issues where surgical accuracy could be increased further.

Acknowledgments The gratitude of the authors goes to the participants who have made this study possible. Furthermore, the authors would particularly like to thank the staff of the radiology department at the BG Trauma Center Tbingen.

Grant Supported by DGUV (Deutsche Gesetzliche Unfallversicherung) FR150.

Compliance with ethical standards

Conict of interest SS is a member of the AO Joint Preservation and Osteotomy Expert Group. DWE is chairman of the United Kingdom Knee Osteotomy Registry, steering committee. The other authors have no conicts of interest to declare.

Ethical approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards.

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