Radiographic classification for fractures of the fifth metatarsal base

Transcription

1 Radiographic classification for fractures of the fifth metatarsal base Running Title: Fracture types of the fifth metatarsal base Alexander T. Mehlhorn MD, Jörn Zwingmann MD, Anja Hirschmüller MD, Norbert P. Südkamp MD, PhD, Hagen Schmal MD, PhD Department of Orthopaedic Surgery, University of Freiburg Medical Center, Freiburg, Germany Corresponding address: Hagen Schmal, M.D. University of Freiburg Medical Center Department of Orthopaedic Surgery Hugstetter Str. 55 D Freiburg i.br. Phone: Fax: Key Words: radiomorphometry fracture fifth metatarsal base - classification

2 ABSTRACT Objective Avulsion fractures of the fifth metatarsal base (MTB5) are common fore foot injuries. Based on a radiomorphometric analysis reflecting the risk for a secondary displacement a new classification was developed. Materials and Methods A cohort of 95 healthy, sportive and young patients (age 50 yrs) with avulsion fractures of the MTB5 was included in the study and divided in groups with non-displaced, primary-displaced and secondary-displaced fractures. Radiomorphometric data obtained using standard oblique and dorsoplantar views were analyzed in association with secondary displacement. Based on this, a classification was developed and checked for reproducibility. Results Fractures with a longer distance between the lateral edge of the styloid process and the lateral fracture step-off and fractures with a more medial joint entry of the fracture line at the MTB5 are at higher risk to displace secondarily. Based on these findings, all fractures were divided in three types: type I with a fracture entry in the lateral third, type II in the middle third and type III in the medial third of the MTB5. Additionally, the three types were subdivided in an A-type with a fracture displacement <2mm and a B-type with a fracture displacement 2mm. A substantial level of interoberserver agreement was found in the assignment of all 95 fractures to the 6 fracture types (κ=0.72). The secondary displacement of fractures was confirmed by all examiners in 100%. Conclusions Radiomorphometric data may identify fractures at risk for secondary displacement of the MTB5. Based on this, a reliable classification was developed. Introduction 1

3 Avulsion fractures of the base of the fifth metatarsal are a common foot injury that causes dorsolateral forefoot pain [1-3]. These injuries have to be clearly distinguished from the true Jones fractures, which is located at the junction between the proximal diaphysis and the metaphysis of the fifth metatarsal involving the 4-5 th intermetatarsal articulation [4] and from the proximal diaphyseal fracture, which is located distally to the 4-5 th intermetatarsal articulation [5]. Avulsion fractures of the fifth metatarsal base occur by indirect force when the foot is plantarflexed and inverted. The exact pathomechanism is controversially discussed, but recently some cadaver studies revealed that the causing force is traction of the lateral cord of plantar aponeurosis and the short peroneal muscle tendon [6, 7]. Therapy recommendation of non-displaced fractures of the fifth metatarsal base includes conservative treatment with protected weight bearing in a walking cast, hard-sole-shoe or a compression wrap [4, 8, 9]. Extended non-weight bearing was identified as a predictor of a poor functional outcome [10]. The therapy of displaced fractures is controversially discussed, both conservative and operative treatment with open reduction, screw fixation or tension band wiring [11-13] is possible [14]. Decision for treatment depends on the patients age, activity level and comorbidities; for healthy and young patients with athletic ambitions operative treatment is more favourable. In the present paper, the hypothesis was tested whether radiomorphometric parameters of fractures of the fifth metatarsal base allow the reliable prediction of a possible secondary displacement. Along with this, a classification was developed considering these observations. Materials and Methods Patients From 2001 through 2009 a cohort of 95 patients treated for avulsion fractures of fifth metatarsal base was included in the study. Only healthy (patients graded 1 or 2 according to the ASA classification [15]), young (age 50 yrs) and sportive patients (at least one sportive activity was performed at time of injury) were included in the present study. All patients considered their sports activities as an essential component for their quality of life. In the same time period a total of 183 patients were operatively treated for metatarsal fractures. Multiple injured patients were excluded, 2

4 4 patients had additional fractures of the foot skeleton. Considering the overall rate of operative treatment following presentation in the emergency department with 70%, the inclusion rate was 40% of all patients with fractures of the fifth metatarsal base treated in the examined time period (95 of 238 patients). After clinical examination, the diagnosis was confirmed by X-rays of the affected foot in a dorsoplantar and an oblique plane. All included observed fractures were proximal to the 4-5 th intermetatarsal articulation in both radiographic planes and were related to an indirect forefoot trauma. The study was approved by the Ethical board. The patients were distributed into three groups: group 1 included patients with non-displaced fractures (step-off < 2mm), who were treated conservatively for 6 weeks. Patients with fractures showing a step-off exceeding 2mm in at least one radiographic plane in initial X-rays were considered primarily displaced and were treated operatively (group 2). Group 3 included patients with initially non-displaced fractures, who were treated conservatively and secondarily displaced (fracture stepoff > 2mm). Following this, they were referred to operative treatment. Conservative treatment consisted of 15kg partial weight bearing for 6 weeks in a lower leg cast (VACOpedes, OPED, Germany). Operative treatment was performed with open reduction and internal fixation using a bicortical screw or tension band wiring followed by 15kg partial weight bearing for 6 weeks in a lower leg cast. Secondary displacement was defined when the initial step-off increased from less to higher than 2mm. During conservative treatment control radiographs were performed 0.5, 1, 2, 4 and 6 weeks after trauma. Radiographic assessment All radiographs were performed in two standard planes including an oblique and a dorso-plantar view by one investigator (A.T.M., 7 years of experience as a dedicated foot and ankle surgeon). Radiographic measurements were performed in the dorso-plantar view using a radiomorphometric analysis tool (Tiani J-Vision, Version , Tiani Medgraph AG, 1150 Wien, Austria/Europe). The lateral fracture step off at the lateral border of the fifth metatarsal base, the tip of the styloid process, the medial fracture step at the metatarso-cuboid joint and the tip of the medial edge of the fifth metatarsal base were defined as the anatomical landmarks. In order to standardize for bone size and to exclude radiographic magnification errors the following proportions and angles were calculated: 3

5 joint fracture angle (JFA) α (Fig. 1, left) joint ratio (JR) C/A (Fig. 1, right) lateral edge length (LEL) B (Fig. 1, right) the lateral edge ratio (LER) B/A (Fig. 1, right) The joint-fracture angle (JFA) α was measured between the proximal articular surface of the fifth metatarsal base and the line connecting the lateral and the proximal fracture step off (Fig. 1, left). The Joint Ratio (JR = C/A) was calculated dividing the line tangential to the metatarso-cuboid joint reaching from the tip of the styloid process to the fracture step-off (C) through the total length of the fifth metatarsal base (A) (Fig. 1, right). The lateral edge length (LEL) was calculated measuring the distance from the tip of the styloid process to the lateral fracture step-off of the lateral surface of the fifth metatarsal (B) (Fig. 1, right). The lateral edge ratio (LER = B/A) was calculated dividing the LEL (B) through the total length of the fifth metatarsal base (A) (Fig. 1, right). Regarding functional anatomy, the LEL and C correspond with the insertion area of the peroneus brevis tendon and the lateral aspect of the plantar aponeurosis [16]. Interobserver reliability and clinical practicability of classification system Based on the results of the radiomorphometric evaluation a classification system was developed, which considers parameters of an increased fracture displacement risk. All X-rays were collected and numerated; patient identification data was rendered anonymously. Then, the X-rays were evaluated by two different observers. Observer 1 was an experienced orthopaedic surgeon with special interest in foot surgery. Observer 2 was a fellow for orthopaedic surgery also with special interest in foot surgery. None of the observers had any experience with the classification before study onset to exclude the influence of training on reliability. At the beginning of the study, the classification was provided to the observers in a written form. In addition, the first author gave a 15minute introductory explanation of the classification. The evaluation of the X-rays by the two observers took place independently. Statistical assessment Numerical data were analysed by computer software package for statistical analysis (SPSS statistical program, version 11.5, SPSS Inc. Chicago, USA). All values are reported as mean ± standard deviation. Statistical significance was determined using the Mann-Whitney U-Test and the Kruskal- 4

6 Wallis H-Test for comparing the medians of two and multiple populations, respectively. Categorial data are presented as absolute frequency and percent distribution. Data (incidences) were arranged in R by C tables, and statistical significance of differences calculated using a 2 test. A significance level of 0.05 was used throughout the study. Percentage agreement and interobserver reliability were assessed with the JMP-statistical package (Version 6, SAS Campus Drive, Building S, Cary, NC, 27513, USA). For intraobserver and interobserver reliability, the kappa statistic function of the JMP-statistical package was used measuring kappa values (ĸ) to describe the agreement between observers [17]. Kappa values were interpreted using the guidelines proposed by Landis and Koch [18]. Results Epidemiological data The mean age of all patients was 39±18 years (41 females and 54 males). 56 fractures, representing group 2, were treated with open reduction and internal fixation (ORIF) with a bicortical screw (n=43) or a tension band wiring (n=13). The remaining fractures (n=39) were initially treated non-operatively. A total of 10 patients, representing group 3, were treated by ORIF following initial non-operative therapy because of secondary displacement. The remaining patients with initial conservative treatment (n=29) represent group 1. All patients were immobilized in a cast as described and performed partial weight bearing for 6 weeks. Secondary displacement was diagnosed within the first 2 weeks of conservative treatment, all fractures consolidated after 3 months. Radiographic assessment The Spearman Rank Correlation revealed a significant correlation between the Joint Ratio (JR) and the Lateral Edge Length (LEL) (ρ=0.63), and the JR and the Lateral Edge Ratio (LER) (ρ=0.65) for all fractures (p<0.0001). The joint-fracture angle (JFA) α was 51±18 in group 1, 55±16 in group 2, and 57±15 in group 3 (Fig. 2A). There was no significant difference between any group. 5

7 In group 1 the JR was 0.48±0.25, in group 2, 0.63±0.26 and in group ±0.16 (Fig. 2B). There was a significant difference between group 1 and 3 (p=0.033) as well as group 2 and 3 (p=0.0004). There was also a significant difference between group 1 and 2 (p=0.018). The LEL was 9.8±4.2 mm in group 1, 12.5±4.2 mm in group 2, and 15.6±2.7 mm in group 3 (Fig. 2C). There was a significant difference between group 1 and 3 (p=0.012) as well as group 2 and 3 (p=0.0006). There was also a significant difference between group 1 and 2 (p=0.009). In group 1 the LER was 0.44±0.18, in group ±0.20, and in group ±0.11 (Fig. 2D). There was a significant difference between group 1 and group 3 (p=0.014) and group 2 and group 3 (p=0.0007). There was also a significant difference between group 1 and 2 (p=0.030). Development of classification system As shown above, a secondary displacement was associated with a higher JR. An association of secondary displacement with the LER could also be demonstrated, but both parameters highly statistically significantly correlated. Therefore, we decided to build up the classification system only on the JR, because the additional parameter LER would not add further information, but complicate the arrangement. The AO classification is based on a 3 class system for all anatomical regions and has been shown to be able to predict outcome [19]. By following this strategy, the total length of the joint surface of the fifth metatarsal base was divided into three equal parts and assigned to three fracture types as follows: type I fractures included fractures of the lateral third, type II fractures of the middle third and type III fractures of the medial third. Type I fractures correlate with a JR 0.33, type II fractures with a JR>0.33 and 0.66, and type III fractures with a JR >0.66. Fractures with a step-off exceeding 2mm were generally considered to influence the clinical outcome [11-13] and defined as displaced. Therefore, we introduced an A-Type (I-IIIA) without a relevant displacement and a B-type (I-IIIB) with a fracture-step-off 2mm (Fig. 3). Validation and interoberserver reliability of the classification system In our study we classified 26% of fractures as Type I (n=24) with a JR of 0.33 or less. Of Type I fractures 50% (n =12) were non-displaced (IA) and 50% (n=12) were displaced (IB). We classified 37% of the fractures as Type II (n=35) with a JR of 0.34 to % (n=14) were non-displaced fractures of Type IIA and 60% (n=21) were displaced Type IIB fractures. We classified 37% (n=35) of fractures Type III with a JR of greater than Type IIIA 25% (n=9) were non-displaced and Type IIIB 75% 6

8 (n=26) were displaced. This classification was a consensus decision of three surgeons including the author. The positive predictive value for a secondary displacement was 0 in Type I fractures (0 of 12 cases), in Type II fractures (1 case of 16), and 0.45 in Type III fractures (9 of 20 cases). Correspondingly, the risk for secondary displacement of initially non displaced fractures is 0% in Type I fractures, 6.25% in Type II fractures, and 45% in Type III fractures. The distribution of risk for secondary displacement is statistically significantly different comparing all groups (p=0.019). This is confirmed calculating the individual risk differences between Type I und III fractures (p=0.01) and Type II and III fractures (p=0.006). If all Type III fractures would have undergone primary operative treatment, 55% would have been stabilized unnecessarily. The JFA was higher in Type I than Type II and III fractures (60±21 vs. 52.5±15 vs. 51.7±14 mm, p=0.033, Fig. 4A). In order to confirm the classification, results for JR are also indicated, although this was the basis for developing the classification types. The JR was significantly higher in Type III fractures compared to Type II and Type I fractures (0.29±0.11 vs. 0.53±0.14mm vs. 0.90±0.09mm, p<0.0001, Fig. 4B). The LEL was significantly higher in Type III fractures compared to Type II and Type I fractures (15±4mm vs. 12±3mm vs. 8±3mm, p<0.0001, Fig. 4C). The Lateral Edge Ratio (LER) was also compared between all three fracture types (Fig. 4D). It was significantly higher in Type III than in Type II and Type I fractures (0.67±0.18 vs. 0.51±0.11 vs. 0.36±0.13, p<0.0001). P-values represent the tests for multiple comparisons; p-values for individual comparisons may be seen in Figure 4. The visually analysed fractures using the introduced classification system revealed in 74% an identical result. A substantial level of agreement (κ=0.72) according to Landis and Koch [18] was found in the evaluation of the different fracture types by observer 1 and 2. The secondary displacement of fractures was confirmed by all examiners in 100%. Discussion Fractures of the fifth metatarsal base are common fore foot injury and are found following indirect trauma. The fracture mechanism is considered to be a fore foot supination with plantar flexion, which leads to an extended muscle force of the peroneus brevis tendon, the metatarsocuboid ligaments and the lateral cord of the plantar fascia at the fifth metatarsal base. Thus, the fracture morphology is strongly dependant on the anatomic predisposition and the type of forces 7

9 affecting the fifth metatarsal base. We analysed a total of 95 fractures of the fifth metatarsal base for epidemiologic and radiomorphometric characteristics as well as for fracture stability during follow up. Based on these findings we developed a new classification for fractures of the fifth metatarsal base considering the risk for secondary displacement. All observations taken together, the largest fragments had the highest risk for secondary displacement. Fractures of the fifth metatarsal base are generally found at all ages and gender. In young patients they have to be strongly separated from the apophysitis at fifth metatarsal base in the growing skeleton [20]. Recent epidemiologic studies on osteoporotic women have shown a high prevalence of fractures of the fifth metatarsal in women older than 65 years and a strong association with a low bone mineral density [21, 22]. In contrast, the introduced analysis focuses more on a population with a high degree of sports activities, which is reflected by the mean age of 39 years. The treatment of fifth metatarsal base fractures ranges from non-operative treatment with a lower leg cast and non-weight or partial bearing for 6 weeks to open reduction and internal fixation with screws or tension band wiring [9, 23-25]. In general, a good healing potential of the fifth metatarsal base is assumed due to its sufficient circulation. Therefore, some authors concluded that metaphyseal fractures not extending beyond the distal end of the 4-5 th intermetatarsal articulation should be treated functionally, regardless of the number of fragments, displacement and intraarticular involvement [26]. Based on the same metaanalysis it was further concluded that only meta-diaphyseal fractures located at the distal end of the 4-5 th intermetatarsal articulation or just distally require early ORIF. This is mainly based on the results of the study by Mologne et al. [27] showing shorter return to sport periods in the intervention group with screw fixation. The conclusion for the proximal fractures of the fifth metatarsal base not extending beyond the distal end of the 4-5 th intermetatarsal articulation is only based on trials comparing different conservative treatment methods [28, 29], but not operative versus non-operative intervention. Especially in a population with a high sports level, early return to physical activities may play a decisive role. Since continuous traction of the peroneus brevis tendon may result in fracture instability with possible development of a non-union, which certainly is a problem [27] in these fractures, the risk for secondary displacement should be reflected by a fracture classification. The highest risk (45%) was found to be associated with Type III fractures with the step-off in the medial part of the fifth metatarsal base. Regarding the risk for development of a posttraumatic osteoarthritis (OA) of the metatarso-cuboid joint some authors strongly recommend operation for displaced, intraarticular fractures [12, 13]. Hereby, a 8

10 fracture step-off exceeding 2mm is widely recognized as a risk factor for OA [30] and has been introduced in the presented classification system. Although classifications inaugurated for this anatomical region by Stewart [31], Lawrence [4] or Holzach [32] sufficiently discriminate between meta-diaphyseal Jones fractures and proximal fractures of the fifth metatarsal not extending beyond the distal end of the 4-5 th intermetatarsal articulation, they all do not reflect the risk for a secondary displacement within the second fracture entity. Therefore, our radiomorphometric analysis of the 95 fractures distinguished between non-displaced, primary displaced and secondary displaced fractures in order to deduce a risk adapted classification. It could be shown that secondary displaced fractures had a longer distance between the lateral edge and the lateral fracture step-off (=LEL). Anatomical studies have shown that the peroneus brevis tendon has a lateral insertion up to 15mm at the fifth metatarsal base [6]. Since the muscle pull of the peroneus brevis tendon is considered as one of the major causes for avulsion fracture mechanisms at the fifth metatarsal base, it is obvious that LELs larger than 15mm increase the risk for displacement. In this case all muscle fibers are attached to the small proximal fracture piece and pull it away more easily, a fact, which could be confirmed in the presented analysis. However, it remains unclear why some fractures primarily displaced and other after a few days. We assume that in secondary displaced fractures the initial fracture energy was lower, but we were not able to clearly confirm this assumption by the histories of our patients. Actually, there are only few studies exactly describing the acting forces as by Orendurff et al. [33] describing the largest bending moments to the fifth metatarsal during acceleration manoeuvres (20±13.1 N/cm 2 ) followed by running straight (11.6±8 N/cm 2 ). Unfortunately, such an exact measurement will not be possible within the setup of a clinical trial. Further, it can be supposed that for initially not displaced fractures a lower leg cast is not able to completely reduce the pulling force of the peroneus brevis tendon allowing the secondary displacement despite cast treatment. Casting the hind-foot in valgus position is maybe an option to reduce the tension of the peroneus brevis tendon. Since avulsion fractures are caused by exceeding muscle force it can be suggested that nondisplaced fractures of the fifth metatarsal base can displace secondarily during conservative treatment. Heineck and collegues [34] found that a reduced activity of the peroneus muscle is critical for adequate treatment and can be achieved by partial weight bearing but not by immobilisation of the talo-crural joint. This is in concordance with the fact that the risk of fracture displacement depends on fracture morphology related to the anatomical onset of the lateral cord of plantar aponeurosis and of the short peroneal muscle tendon. 9

11 A second risk factor for a secondary displacement was a high joint ratio (JR); a high JR stands for a medial fracture entry into the metatarso-cuboid joint. As recent studies revealed, another avulsion force for fractures of the fifth metatarsal base is mediated by the lateral aspect of the plantar aponeurosis and of the plantar metatarso-cuboidal ligaments [6, 7]. From this point of view it could be assumed that a fracture portion with a more medial proximal entry within the metatarso-cuboid joint is more unstable, because the fracture does not run through the middle of the plantar ligamentous structures but medial of its main cord. We additionally could show that fractures with more medial joint entry are associated with a longer Lateral Edge Length (LEL), which destabilizes the distal fracture piece additionally as discussed above. According to these findings we developed a classification system based on the JR and the fractures step-off. The last criterion has already been considered as a general prognostic criterion for fifth metatarsal fractures [30]. The lateral edge length (LEL) was not directly included in the classification; however, since the LEL and the lateral edge ratio (LER) statistically significantly correlated with the joint ratio (JR), both were indirectly taken into account. Further, it was shown that LEL and LER were significantly higher in Type III than in Type II and I fractures. The clinical practicability and interobserver reliability of the classification system were tested by two surgeons with different experience level and no experience with the classification before study onset. A substantial level of agreement was found in the assignment of all fractures to the different fracture types of our classification system. Especially, a definitive distinction between fractures of Type II and III seems to be crucial since later therapy decision might depend on this correct classification. Therefore, we recommend an exact radiomorphometric analysis (digital or manual e.g. using a ruler or an electronic measurement device) in case of doubt. Based on the average length of the fifth metatarsal base of 22mm the measurement accuracy should be between 1mm and 2mm considering a confidence interval less than 10%. Overall, the suggested classification was only examined for reproducibility and needs to be further validated in a separate data set. The limitations of this study are the small number of cases in group 3, which included the secondarily displaced fractures. Despite, all cases were assessed by two independent observers as secondary displacements and differences concerning the radiomorphometric data were statistically significant. The informative value of the study is further restricted by the limited number of observers. 10

12 Summarizing the results of this study, at first radiomorphometric characteristics of fifth metatarsal base fractures were evaluated identifying risk factors for a secondary displacement following initial conservative treatment. Based on this, a new classification system dividing the articular surface into thirds was introduced demonstrating good interoberver reliability. Considering a fracture step-off larger than 2mm as a risk factor for a posttraumatic OA, open reduction and internal fixation (ORIF) of all Type B-fractures may be recommended. However, a fracture treatment remains individually decided and is dependent on general health conditions and functional demands. Furthermore, all initially non-displaced fractures with a more medial joint fracture step-off (Type IIIA) may - considering the same circumstances be recommended for ORIF because of its risk for secondary displacement. At least the risk of 45% should be part of the discussion with the patient about treatment risk when deciding for conservative treatment. If non-operative treatment is preferred, fractures at risk for secondary displacement should undergo frequent X-ray controls within the first 2 weeks, in case a secondary displacement would trigger the decision for ORIF. Legends Figure 1 Overview about the evaluated radiographic parameters used for analysis. Radiographic measurements of all fifth metatarsal base fractures were performed in the dorso-plantar view. The joint-fracture angle (JFA) α, the Joint Ratio (JR = C/A), the lateral edge length (LEL=B), and the lateral edge ratio (LER = B/A) were calculated based on these measurements. Figure 2A-D The patients were distributed into three groups: group 1 included patients with non-displaced fractures (step-off < 2mm), who were treated conservatively. Patients with fractures showing a stepoff exceeding 2mm in at least one radiographic plane in initial X-rays were considered primarily displaced and were treated operatively (group 2). Group 3 included patients with initially nondisplaced fractures, who were treated conservatively and secondarily displaced (fracture step-off > 11

13 2mm). The joint fracture angle α (shown in part A), the ratio of base length/basal fracture length (JR, shown in part B), the lateral edge length (LEL, shown in part C), and the ratio of the lateral edge length and base length (LER, shown in part D) were measured, calculated and compared between the different groups. All values are reported as mean ± SD. Results for statistical comparisons are indicated in the figures. Figure 3 A classification system was developed based on an increased risk for secondary displacement of fractures with a more medial joint entry of the fracture line at the fifth metatarsal base. Dependant on the joint entry of the fracture line at the fifth metatarsal base (lateral one-third, middle one-third and medial one-third) Type I, II or III were defined. Fractures without displacement were summarized as A-Type (I-IIIA) and with a fracture-step-off >2mm as B-Type (I-IIIB). Figure 4A-E The radiomorphometric parameters joint-fracture angle α (JFA, shown in part A), ratio of base length/basal fracture length (JR, only shown in part B to confirm accuracy of classification), lateral edge length (LEL, shown in part C), and the lateral edge ratio (LER, shown in part D) were compared between all three fracture types. All values are reported as mean ± SD. Results for statistical comparisons are indicated in the figures. Part E demonstrates an example for secondary displacement of a Type III fracture. 12

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