Application of the Modified RUST Score in Tibial Bone Transport and Factors Associated with Docking Site Complications

INTRODUCTION

Complex tibia fractures are common injuries and bone defects following open fractures or fracture-related infections are challenging to treat.¹ Distraction osteogenesis (bone transport) using a circular external fixator, as described by Ilizarov, is a well-established treatment option for the reconstruction of segmental bone defects.^1–3

Mechanical complications following bone transport include refracture (4–21%), malunion (4–22%),^4–6 axial deviation (22–70%),⁶ non-union (0–40%)^7–9 and callus subsidence.^10,11 These complications can be related to both the transport site and the docking site. A recent retrospective study on 103 patients noted a complication rate of 53% at the docking site.⁸ Premature removal of the external fixator is one of the factors that has been associated with an increased rate of non-union at the docking site.⁸

Although distraction osteogenesis is a well-known treatment, especially in low and middle-income countries,¹² the safe timing of frame removal to avoid such complications is still poorly defined.¹³ Several methods have been described to assess the transport site (the regenerate bone), including a radiograph-based classification,¹⁴ as well as pixel value ratio and DEXA.^15–18 On the other hand, there is currently no specific tool for assessing the union at the docking site. Given the high rates of complications reported at the docking site, particularly in terms of mechanical complications, guidelines for the safe and timely removal of the frame following bone transport using simple and inexpensive assessment, such as standard X-rays, would be of clinical value.

The RUST score – Radiographic Union Score for Tibial fractures – developed by Whelan et al.,¹⁹ then modified by Litrenta et al.,²⁰ is a simple rating system based on standard X-rays. The modified RUST (mRUST) consists of scoring each cortex on the anteroposterior and lateral radiographs on a 4-point scale. The final score is the sum of the four cortices, with a value from 4 to 16. It has been demonstrated that the score is well correlated with bone healing and biomechanical properties in animal models.^21–23 A cut-off of 11 points is recognised as the minimum threshold for union, and 13 points as definite union.^20,21 The modified RUST has been shown to have good interobserver reliability^20,23 and has been used in various clinical settings,^24–27 including tibial shaft fractures with bone defects.²⁸ This latter study is, to our knowledge, the only one to include some patients with a circular external fixation frame. While the study did not look at bone transport specifically, it did highlight the potential problem of applying the mRUST score in this situation, due to the frame elements obscuring the view of one or more cortices.

To our knowledge, the mRUST score has never been applied to the docking site in the setting of bone transport with a circular frame. It could represent a simple tool to help determine the optimal time for safe frame removal with reduced risk of complications. We therefore aimed to assess the application and the interobserver reliability of the mRUST score in bone transport, in an adapted manner, to account for the potential obstruction by the frame. Secondarily, we aimed to identify factors associated with an increased risk of docking site complications.

METHODS

This retrospective study was conducted at a single tertiary centre in South Africa. Ethical approval was obtained from the relevant ethics committee prior to conduction of this study (protocol reference number BREC/00006615/2024). Between 2014 and 2023, 29 patients underwent a tibial bone transport using a circular external fixation frame. Patients older than 13 years were included if complete radiological follow-up with the frame was available. For the analysis of complications, only patients with at least one additional follow-up visit after frame removal were retained. After reviewing the radiological records, five patients were excluded, three due to incomplete radiological records, and two due to not having completed their treatment at the time of the analysis. The remaining 24 patients were included in the study. Another patient was excluded from the complications analysis because no radiological follow-up was available after frame removal.

Demographic data (age, sex) and baseline bone transport characteristics were recorded as follows: we included the location and size of the bone defect at the time of frame application; the presence or absence of a cement spacer prior to transport frame application; the site of the distraction (proximal, distal or trifocal); the distraction regenerate size; the duration of bone transport; the total duration of the frame as well as the external fixator index (EFI) and the follow-up duration for each patient. The configuration of the docking site was described in terms of the relationship between the two bone ends and the angulation (collinearity) between their surfaces on anteroposterior and lateral views was recorded, with 0° corresponding to the two surfaces being parallel to each other (Fig. 1). This measurement was made on the postoperative radiographs providing the best views of both bone ends, taken within two months following the application of the transport frame. Complications occurring after frame removal, either at the docking site or at the transport site, were recorded, based on the radiological file, and defined as follows: fracture, non-union, any angulation >5°, shortening >5 mm, or any other complication requiring reoperation.

RESULTS

The study population consisted of 20 males and 4 females, with a median age of 26 years. The most common defect location was the middle third of the tibia and the most common type of distraction was bifocal transport, from proximal-distal. The median EFI was 67 days/cm and the median follow-up after frame removal was 112 days. Demographic and detailed transport characteristics are shown in Table 1.

The overall rate of mechanical complications was 21.7%, occurring in five patients (Table 2). All complications were related to the docking site and three patients required reoperation. These three patients all underwent intramedullary tibia nailing. The first patient had a 16° valgus angulation occurring 3 weeks after frame removal (Fig. 3). The second patient sustained a docking site fracture two years after the removal of the frame, and the third had a persistent non-union at the docking site. The two remaining patients with complications were treated non-operatively. The first had a gradual valgus deformity which never progressed beyond 7°, and the other, lost to follow-up for 1-year post removal, presented with hypertrophic callus at the docking site and signs of previous fracture, and was again lost of follow-up before management of this hypertrophic non-union.

The 5 patients who developed complications were compared to the 18 patients without complications (Table 3). No statistical difference was found in demographics and bone transport characteristics, except for the length of time of the follow-up. The mean mRUST ratio was not statistically different between the two groups, on radiographs before and after frame removal. For both groups, the mean mRUST ratio was above the minimum threshold for union, namely 0.69. The mean number of cortices seen on X-rays with the frame was 2.96 and 3.40, for the group without complications and with complications, respectively. Therefore, the radiological assessment of the docking site was not made more difficult by the potential obstruction of the cortices by the frame in the patients who had complications.

For the entire study population, the mean number of visible cortices was 3.0. The mean mRUST ratio was 0.70 with the frame and 0.77 without the frame. The correlation of the mean mRUST ratio before and after frame removal showed a moderate relationship, with a Spearman correlation coefficient of 0.50 (p-value 0.13).

The results of the inter-rater reliability analysis of the mRUST ratio, and the scoring for each cortex individually, are shown in Table 4. The agreement between the three appraisers is “slight” for the number of cortices that could be scored, with a Fleiss Kappa of 0.033. Overall, the agreement is “fair” (Kappa 0.21–0.40) for the scoring of individual cortices, apart from one score, which is “slight” (Kappa 0.00–0.20). In contrast, the inter-rater reliability of the mRUST ratio shows higher agreement than the individual cortex scores, with an ICC of 0.662 with the frame and 0.759 without the frame.

Post-hoc analysis revealed that the only factor associated with an increased risk of complications at the docking site was the configuration of the docking site, represented by the angulation of the bone ends (Table 3). After analysing different cut-offs (≥ 30°, 45° or 60° on at least one of the anteroposterior or lateral views), we found that 45° represented the best cut-off angle related to the presence or absence of complication, (p < 0.001 with a post-hoc power of 97.9%).

DISCUSSION

Distraction osteogenesis is a well-recognised treatment method to address lower limb bone defects, and is often the only option for limb salvage, despite a potentially high rate of complications. In our study, we found a rate of 21.7% of mechanical complications and all were related to the docking site.

To date, there is no real consensus on the best approach to avoid docking site complications, as highlighted by Giotakis et al.³⁵ Several management options have been advocated: the accordion technique, percutaneous osteotomy of bone ends, removal of soft tissue interposition and bone grafting.³⁵ Performing a routine docking revision procedure is not ubiquitous. However, according to Tetsworth et al.,³⁶ most authors currently recommend this procedure. A recent systematic review³⁷ including 23 studies and 1,153 patients showed a significant difference in docking site union with a union rate of 90% if a planned docking procedure was performed, versus 66% in the group without. In the seven included studies reporting planned docking procedure, this procedure consisted of refreshing of bone ends and/or autologous bone grafting. However, the heterogeneity of the studies remains a limitation in drawing conclusions about the optimal approach.³⁷ Bone transport through an induced membrane may possibly improve the chance of docking site union, as shown in the randomised controlled trial (RCT) by Thakeb et al.,⁹ including 30 patients, who found that docking site non-union was significantly lower in the induced membrane transport group (0%), negating the need for a docking procedure, compared to 40% in the other group.

Our study showed a lower rate of mechanical complications at the docking site compared to Feng et al.,⁸ who reported 21.4% delayed union, 18.4% axial deviation and 9.7% soft tissue incarceration rates. Interestingly, a formal docking site grafting was not performed as a routine procedure in this study. Distal third tibia bone defects as well as the overall defect size were highlighted as independent risk factors for delayed union. These factors were not associated with complications in our study. Spiegl et al.³⁸ in their series of 24 patients, with a formal docking procedure performed routinely, described one non-union and four axial deviations, without specifying whether these deviations occurred at the docking site or at the transport site. Liu et al.³⁹ in their large retrospective study including tibia and femur bone transport, reported 19% axial deviation, 19% delayed union and 8% refractures of docking sites amongst their major complications. A narrative review by Aktuglu et al.⁴ analysed a total of 27 studies and only 3 of these reported to be doing routine docking procedures. However, overall 40.1% of patients required docking site grafting after completion of the transport due to delayed- or non-union.

In our study, all patients underwent a formal docking procedure and a transport through an induced membrane. Mechanical complications at the docking site were recorded in more than 1 in 5 (21.7%) patients. This complication rate is comparable to rates reported in the literature. However, the comparison is limited due to the heterogeneity of docking site management in other studies. In our experience, the docking site is much more often a source of complications than the regenerate site.

As there are no clear recommendations in the literature regarding the optimal time to remove the frame, we evaluated the application of the mRUST to the docking site to assess whether this score could be relevant in deciding whether it is safe to remove the frame to avoid mechanical complications. We found no difference between the two groups with or without complications in terms of the mRUST score. The threshold for union was achieved before the removal of the frame in both groups. Therefore, the mRUST does not appear to be clinically useful in predicting the occurrence of mechanical complications after frame removal. This result could be explained by the fact that the docking site does not have the same biological behaviour as a fracture. It has been shown that while the docking site does not differ histologically from the normal process of indirect or secondary consolidation of a fracture, with endochondral ossification, this process occurs in a slower manner.⁴⁰ In addition, the initial gap is gradually filled by fibrocartilaginous tissue, creating a situation similar to non-union, and in some cases, soft tissue can be interposed at the docking site during the transport process.³⁵

It should be noted that an easy way to assess the healing of the docking site would be to perform a CT scan, but this modality is not widely available in low- or middle-income countries due to resource constraints. In our setting, the waiting time for a non-urgent CT scan is more than 6 months and could therefore not be routinely used.

The inter-rater reliability of the mRUST, applied to individual cortices at the docking site, was only “fair” or even “slight”, considering the Fleiss Kappa and KA coefficients, which are appropriate for the assessment of ordinal data. Despite our ICC values being comparable or even higher than previous studies, evaluating the inter-rater reliability of the RUST or mRUST score,^{20,21,23,26,33} we believe that the values of Fleiss Kappa and KA coefficients should be considered instead of the ICC. In fact, assessing the inter-rater reliability of ordinal data in the same way as continuous data may lead to overestimation of the reliability. This is because the analysis will show better agreement when using the ICC since the number of scoring possibilities is supposedly unlimited. However, in the case of the mRUST score, only four different scores are used. This difference in results between ICC and KA coefficient is also shown in the study by Mitchell et al.²⁸ on the reliability of the mRUST score in tibial fractures with bone defects. To the best of our knowledge, this is the only study that has used the KA coefficient for this purpose. In their study, the differences between the two methods of analysis are smaller than in our results. This could be explained by the larger sample size compared to our study.

For the mRUST ratio, we used the ICC because the ratio is a continuous variable. However, as the ratio is the result of four ordinal measurements, this result should be treated with caution. Nevertheless, the finding of relatively high ICC values for the mRUST ratio (0.66 with the frame and 0.76 without the frame) can be explained by the fact that the discrepancy in scoring for individual cortices is reduced when the scores of the four cortices are considered together, (i.e., one reviewer scored two for the anterior cortex, three for the posterior cortex and another reviewer scored the opposite). Mitchell et al.²⁸ hypothesised that the level of reliability may be influenced by the severity of the original injury in patients treated with external fixation and the presence of a previous bone graft. This, in turn, may lead to an abnormal appearance during the healing process, thus explaining the lower level of agreement. We agree with this hypothesis and believe that the low level of reliability found in our study may be related to the inherently different nature of a docking site, that is, the abnormal bone ends with a biomechanical environment that is different from a normal fracture, and the presence of bone graft.

In light of our results, and the reasons given above, we do not consider the mRUST to be a reliable tool for assessing union at the docking site. However, its application to a larger study population is required to confirm this finding. Our findings suggest that the application of the mRUST score, as a ratio, could allow the score to be used despite cortices hidden by the frame, as the mean number of cortices seen with the frame was 3, and the correlation of the score with and without the frame was moderate. Therefore, the mRUST ratio could be considered to evaluate the healing of fractures treated with a circular frame and could overcome the difficulty in scoring reported by Mitchell et al.²⁸ when one or more cortices are hidden by the frame. A study evaluating the mRUST ratio in tibial fractures treated with a circular frame should be performed to confirm this assumption.

Therefore, the question of when it is safe to remove the frame remains unresolved. Some authors consider a solid docking site when corticalisation or bridging callus is achieved in three of four cortices,^38,39,41 while others only mention that the frame is removed when the docking site has healed without further details.^42–44 A “destabilisation” of the frame before removal is often described,^{7,9,39,41,42,44,45} as well as a cast for 4 to 6 weeks after frame removal.^7,9,39,41 Our standard practice is to destabilise the frame for 2 weeks before removal, to ensure the absence of pain and/or radiographic displacements. Once the frame has been removed, the patient is allowed to full weight-bear without a cast or splint. As previously mentioned, our rate of mechanical complications is not higher than that reported in the literature.

An important point found in the post-hoc analysis of this study is the relevance of the physical configuration of the docking site in terms of the amount of bone contact and the orientation of the surfaces of the bone ends meeting at the docking site. There is a statistically significant difference in the angulation of the bone ends between the groups with and without complications, with a cut-off of 45°. This may have important clinical implications when considering the extent of the bone debridement. Although it may seem logical that a larger contact area offers a better chance of healing, the concern to preserve as much bone stock as possible when dealing with a bone defect can result in irregular bone ends or an oblique cut after debridement. Recommendations regarding the shape of bone ends in the context of bone transport are scarce in the literature. Liodakis et al.³⁷ mentioned that bone contact is influenced by the geometry of the bone cuts at initial debridement and by the accuracy of the alignment, while Giotakis et al.³⁵ reported that some surgeons have advocated a plunger and mortise shape to increase the contact and to eliminate the interposed fibrous tissue. In the present study, we found a high correlation between the occurrence of mechanical complications at the docking site and a divergence in the angle between the bony surfaces of 45° or more. It remains unclear if obtaining two parallel bone ends is more important than, for example, perfectly perpendicular cuts relative to the bone axis. Considering our results, we recommend that further research has to be done to determine the optimal shape of the bone ends to be docked.

The limitations of our study are the relatively small number of cases, the inherent limitations of a study based on radiological records and the relatively short follow-up. The latter is explained by the fact that access to health care can be a challenge in our setting. As a result, follow-up is terminated as soon as clinical and radiological results are deemed satisfactory. Despite these limitations, the main point highlighted by this study, that is, the importance of the shape and orientation of the bone ends at the docking site, can be considered as a valuable finding due to the high post-hoc power.

CONCLUSION

This study did not find the mRUST or mRUST ratio useful in assessing the healing of the docking site to help decide on the best time to remove the frame. However, a notable finding was that the shape and orientation of the bone ends meeting at the docking site might well be relevant to decrease complication rates. If the angle between the bony surfaces is 45° or more, it may be associated with an increased risk of complications as seen in our cohort. It may be worthwhile considering reshaping these bone ends at the time of debridement or formal docking procedure to be more collinear, in order to reduce the potential for mechanical complications such as non-union, axial deviation or refracture at the docking site.

ORCID

Leonard Charles Marais https://orcid.org/0000-0002-1120-8419

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