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The Rotterdam Foot Classification A Classification System for Medial Polydactyly of the Foot

Elise B. Burger, MD, Steven E.R. Hovius, MD, PhD, Bart J. Burger, MD, PhD, and Christianne A. van Nieuwenhoven, MD, PhD

Investigation performed at Erasmus Medical Centre, Rotterdam, the Netherlands

Background: Polydactyly at the medial side of the foot (“medial polydactyly” of the foot) is a rare and diverse congenital anomaly. In order to plan and evaluate surgical treatment, the classification of medial polydactyly is useful. The aim of our study was to develop a reliable and valid classification system for medial polydactyly of the foot that is more useful than previous systems for preoperative evaluation and surgical planning. Methods: A review of the literature and the clinical experience of a single experienced surgeon were used to determine classification categories. We identified all patients with medial polydactyly who had preoperative radiographs and clinical photographs and were treated at our hospital between 1993 and 2014. All affected feet were assessed according to our proposed classification system, the Rotterdam foot classification. The intrarater and interrater reliability among 5 ob- servers who evaluated 30 feet were assessed with use of the Cohen kappa (k) statistic. Results: We developed a classification system that describes duplication type, syndactyly, the presence of a hypoplastic ray, and deviation of the hallux. Seventy-three feet were classified according to the system. Seven duplication types were distinguished. Complete metatarsal duplication was most frequently seen (in 29%). Twelve feet showed a broad hallux without external expression of duplication. Syndactyly between medial and lateral (duplicate) halluces was present in 30 feet; between the lateral hallux and second toe, in 13 feet; and between both duplicated halluces and the lateral hallux and second toe, in 21 feet. A hypoplastic ray was seen in 75% of the feet. Intrarater agreement for duplication, hypoplastic rays, syndactyly, and deviation were, respectively, k = 0.79, 0.75, 0.59, and 0.78. Interrater agreement for duplication, hypoplastic rays, syndactyly, and deviation were, respectively, k = 0.72, 0.54, 0.48, and 0.64. Conclusions: The proposed classification system contains 4 categories of anatomic features of the foot. Classification of all categories shows moderate to good reliability. Use of the Rotterdam classification in evaluating medial polydactyly improves type-specific description, which may, in the future, enhance the evaluation of surgical treatment. Clinical Relevance: The Rotterdam foot classification system is a reliable and easy-to-use system that we believe will improve communication between clinicians and researchers and facilitate the evaluation of treatment results in medial polydactyly of the foot.

Peer review: This article was reviewed by the Editor-in-Chief and one Deputy Editor, and it underwent blinded review by two or more outside experts. The Deputy Editor reviewed each revision of the article, and it underwent a final review by the Editor-in-Chief prior to publication. Final corrections and clarifications occurred during one or more exchanges between the author(s) and copyeditors.

olydactyly is a congenital malformation characterized by Medial polydactyly of the foot is not extensively studied, extra digits of the hands or feet and can be described as and cohorts are usually small. A reason for the deficitinthe P 1 preaxial, postaxial, or central polydactyly . A report on the literature may be the low prevalence and the relatively minor prevalence in the Netherlands showed 8.4 patients per 10,000 related functional problems2-4. However, the hallux is important births with polydactyly, with only 0.4 patients per 10,000 births for pressure distribution and directional control during walking 5-7. with preaxial polydactyly of the foot, also known as medial Furthermore, Phelps and Grogan reported the necessity of treat- polydactyly (involvement of the medial side of the foot). Forty ment in most patients with polydactyly due to shoe-fitting prob- percent of these patients were diagnosed with a syndrome2. lems and an unsatisfactory aesthetic appearance8. Consequently,

Disclosure: Funding was received for this study from the Esser Foundation, which is associated with the Department of Plastic, Reconstructive and Hand Surgery, Erasmus Medical Centre, Rotterdam, the Netherlands. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article.

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TABLE I Present Classification Systems for Polydactyly of the Foot

Classification System Limitations

Watanabe et al.14 (1) Uniform classification of different subtypes is difficult. Tarsal type (with 1 subtype) (2) Subtypes are specific to that patient population Metatarsal type (with 3 subtypes) Proximal phalangeal type (with 5 subtypes) Distal phalangeal type (with 6 subtypes) Seok et al.12 (1) Combination of lateral and medial polydactyly in 1 classification system. S0: no syndactylism (2) No description of polydactyly of the phalanges S1: incomplete syndactylism S2: complete syndactylism A0: angulation of <15° A1: angulation of 15°-30° A2: angulation of >30° M0: no metatarsal extension M1: wide metatarsal head M2: metatarsal shaft not shared (complete or incomplete) Venn-Watson4 (1) No description of polydactyly of the distal phalanges. Complete duplication of the metatarsal (2) Classification possible only for duplication level Wide metatarsal head Short block metatarsal Y metatarsal T metatarsal Masada et al.13 (1) No description of differences between polydactyly of the distal and the Type 1: ray duplication proximal phalanx. fi Type 2: completely duplicated phalanges (2) Classi cation possible only for duplication level Type 3: incompletely duplicated metatarsal Type 4: incompletely duplicated phalanges Blauth and Olason15 (1) Combination of lateral and medial polydactyly in 1 classification system. Tarsal type (2) Classification possible only for duplication level Metatarsal type Proximal phalangeal type Middle phalangeal type Distal phalangeal type With addition of the digit localization, indicated by a Roman numeral (I-V)

the preservation of foot function and the reduction of shoe-fitting Content validity refers to the extent to which a classification problems with medial polydactyly are challenging and require represents all important factors of a condition. When items are individualized treatment based on the anatomical and clinical selected carefully and within reason, the content validity is higher. appearance of the foot. The use of a clear, uniform classification Construct validity refers to the extent to which a test measures the system will result in simplified communication and improved construct it claims to be measuring. For example, when a clas- comparison of different features9,10. sification system is unable to distinguish between different types, Ideally, classification systems can help to guide manage- it has a low construct validity. Criterion validity refers to the ment of treatment and provide prognosis. However, developing degree to which the classification correlates with other measures aclassification system is challenging. The system should be easy or outcomes. For example, if the classification corresponds to to use and allow for adaptation and extension of the system9. treatment results, then it has a high degree of criterion validity 11. Furthermore, it must be valid and reliable11. Reliability is easy to Unfortunately, current classification systems for medial measure with an analysis of interrater and intrarater agreement. polydactyly do not fulfill the important factors for a good clas- Validity is a broader concept, with content validity, construct sification system (Table I). The comparison of feet of different validity, and criterion validity being the most important types. phenotypes is difficult with some classification systems because 1300

T HE J OURNAL OF B ONE &JOINT SURGERY d JBJS. ORG THE ROTTERDAM F OOT CLASSIFICATION VOLUME 98-A d NUMBER 15 d AUGUST 3, 2016 of the impossibility of classifying all types. For example, the classifications of Seok et al.12 and Venn-Watson4 do not describe TABLE II Terms Extracted from Literature Review fi polydactyly of the distal phalanx. Likewise, by the classi cation No. of Articles system of Masada et al., no distinction between polydactyly Extracted Term Describing Term of the distal or proximal phalanx is made, which also results in difficulty in classifying feet13. Moreover, Watanabe et al. de- Duplication level* 22 scribed medial polydactyly on the basis of their own patient Ray involvement 12 population and distinguished between tarsal, metatarsal, proxi- Valgus deformity/deviation* 12 mal phalangeal, and distal phalangeal types14. This resulted in 15 Syndactyly* 6 groups for medial polydactyly that were specifictothatpopula- Hypoplastic ray/rudimentary ray* 4 tion and without a clear analogy among the different types. The Triphalangism 3 drawings used to describe specific characteristics of the feet, such Polysyndactyly 3 as hypoplastic rays and deviation, lack universal properties and are not easy to use. Furthermore, medial and lateral polydactyly of Triplication 3 the foot are sometimes grouped together in classification systems; Floating hallux 2 both Blauth and Olason15 and Seok et al.12 combined medial and Mirror foot 2 lateral polydactyly in 1 classification system. We think that this Delta phalanx 2 decreases the content validity of the classification of medial pol- Shape of the metatarsal 1 ydactyly because Venn-Watson described a difference in the an- Rotation 1 atomical properties of medial polydactyly compared with lateral Vascular and nerve deformities 1 polydactyly4. In addition, it is known that the hallux has a more Upper and lower-limb deformities 1 importantfunctionthanthefifth toe, which may also result in a 5,6 decrease in criterion validity . *Terms selected for the Rotterdam foot classification. We believe that the present classification systems do not provide a comprehensive description of medial polydactyly. Therefore, the aim of this study was to develop a more valid, reliable, and easy-to-use classification system. In order to improve Five raters, who included 2 plastic surgeons (S.E.R.H. and C.A.v.N.), 1 ortho- fi paedic surgeon (B.J.B.), 1 medical student, and the principal investigator, the content validity of this new classi cation, we performed a classified the presented clinical and radiographic images according to the devel- literature review and held a consensus meeting to determine all of oped classification system. The principal investigator was excluded in the intra- the important contributing factors to be considered in the clas- rater agreement analysis because we expected a biased outcome due to multiple sification of medial polydactyly. Next, we tested the usability of the case views by the principal investigator during the study. We chose to involve the developed descriptive classification system by assessing our own 2 specialties because patients consult both plastic and orthopaedic surgeons. population, and an agreement analysis was performed among 5 Radiographs and clinical photographs were presented via LimeSurvey, a protected fi computer program used for questionnaires and assessments. Two classification observers to test the reliability of the classi cation system. rounds were performed within approximately 4 to 6 weeks. Raters performed the classification independently and were blinded to the clinical information of the Materials and Methods patients. Development of the Classification System The intrarater and interrater agreement was assessed with use of the k n Embase literature search (see Appendix) for classification systems for Cohen kappa ( ) statistic. Guidelines of Landis and Koch were used for in- k 16 k polydactyly of the foot was performed in 2014, in order to develop a rep- terpreting values . The value of intrarater agreement was calculated using A fi fi resentative list of contributing factors of medial polydactyly. The article titles and the classi cation for each pair of observations of the 4 observers in the rst and k abstracts were reviewed to identify studies about classification systems. All clas- second rounds and then averaged to provide a single value. The average sifiable aspects of medial polydactyly were extracted and reviewed by an experi- percentage of agreement among the observers was also calculated. Interrater fi fi enced plastic surgeon (C.A.v.N.) and the principal investigator (E.B.B.). Relevant agreement was calculated by comparing the rst-round classi cations among k categories were chosen on the basis of occurrence of the category in the literature the different raters. Again, an average of and the percentage of agreement were fi and the influence on function and aesthetic outcome according to the literature. calculated. Because no gold standard for the classi cation was present, the agreement among the observers was analyzed using the classification that was k Classification of Our Population most chosen by the 5 observers as the gold standard. Statistical analyses of values were performed using SPSS software (version 22.0; IBM). The per- We searched the hospital database of the congenital hand team of our de- centages of agreement were calculated using Excel software (2010; Microsoft). partment for patients with medial polydactyly of the foot seen between 1993 The study was approved by the Institutional Ethical Review Board of the and 2014. Patients with Apert syndrome were excluded from this initial search Erasmus Medical Centre, Rotterdam (mec-2014-263), and is in accordance because foot anatomy is evidently different in these patients. Patients without with the Declaration of Helsinki. preoperative radiographs and clinical photographs were also excluded. Foot anomalies were classified according to the new classification system by the principal investigator (E.B.B.). The occurrence of each type was analyzed. Results Development of the Classification System Agreement Analysis he search of the literature resulted in 650 articles. After our An analysis was performed in order to test intrarater and interrater agreement. Treview of the titles and abstracts, 46 articles remained. We The principal investigator (E.B.B.) randomly chose 30 feet from our database. compiled a list of contributing factors that were mentioned and 1301

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syndactyly. In cases in which no syndactyly was present, no “S” TABLE III Patient Characteristics* classification was made. “ ” No. of Patients (%) A hypoplastic ray is abbreviated as the letter H fol- lowed by “M” or “L,” representing a medial or lateral hypo- Sex plastic structure. In the classification of a hypoplastic ray, all Male 16 (37%) hypoplastic osseous structures are taken into account. In Female 27 (63%) addition, a preaxial ray with both lateral and medial hypo- Affected foot plastic structures can be scored as HLHM. In cases in which Left only 4 (9%) no hypoplastic structure was present, no “H” classification Right only 9 (21%) was made. “ ” Bilateral involvement 30 (70%) Deviation of the hallux, abbreviated as the letter D, is noted when the alignment of the hallux, which we determined Lateral foot polydactyly 12 (28%) on the basis of clinical photographs, is not in alignment with Syndrome diagnosis 25 (58%) the rest of the foot.

*N = 43 patients (73 affected feet). Classification of Our Population A total of 64 patients with medial polydactyly of the foot were identified from the hospital database. Twenty-one patients the number of articles describing each (Table II). Ray in- were excluded; 11 patients did not have preoperative radio- volvement was the second-most mentioned factor in the liter- graphs because of conversion to electronic patient systems, ature; however, our classification system is exclusively for 3 did not have clinical photographs, and 7 did not undergo polydactyly of the first ray, so we excluded this factor. Con- surgery in our hospital. The available radiographs or clinical sensus was reached regarding the other 4 most mentioned photographs of these 21 patients were reviewed, and no foot categories: duplication level, syndactyly, hypoplastic ray, and deviation. Duplication level was included because the surgeon TABLE IV Distribution by Classification Category* should be informed about duplicated osseous structures. Syn- dactyly was included because planning for incision and the No. of Feet (%) need for tissue grafting are influenced by the presence of syn- dactyly. Furthermore, expected deviation after surgery depends Duplication on the presence of syndactyly between the preserved hallux and 0 1 (1%) second toe. Hypoplastic ray was added to the classification I 11 (15%) because the choice of excision side may be influenced by the II 18 (25%) presence of a hypoplastic ray. Deviation of the hallux was in- III 0 (0%) cluded because it influences the surgical techniques used to IV 19 (26%) achieve correct orientation of the hallux. As the classification V 1 (1%) system is only for medial polydactyly, involvement of rays other VI 21 (29%) than the first ray was not included. VII 0 (0%) The proposed classification system is illustrated in Figure VIII 2 (3%) fl 1. The appearance of a oating hallux or polydactyly without Syndactyly fi osseous structures is classi ed as type 0. The duplication type is No 0 (0%) scored by Roman numerals, corresponding to the Rotterdam S0 12 (16%) classification system for radial polydactyly and initially derived S1 30 (41%) from the Wassel classification, starting with type I, for distal S2 13 (18%) phalangeal duplication, and proceeding to type VIII, for dupli- S1S2 18 (25%) cation of the tarsal bones17,18. Even numbers represent complete duplication of the osseous structure, and odd numbers represent Hypoplastic ray incomplete duplication. No 18 (25%) Syndactyly, abbreviated as the letter “S,” is classified as S0, HL 34 (47%) S1, S2, or S1S2. S0 represents the presence of a broad hallux, HM 21 (29%) without the appearance of syndactyly. S1 indicates the presence Deviation of syndactyly between medial and lateral (duplicate) halluces. No 20 (27%) S2 indicates the presence of syndactyly between the lateral D 53 (73%) hallux and the second toe. S1S2 indicates syndactyly between duplicate halluces and between the lateral hallux and the second *N = 73 feet. toe. No distinction was made between complete or incomplete 1302

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Duplication

Fig. 1 The Rotterdam foot classification, a proposed classification for medial polydactyly. The type of duplication is indicated by Roman numerals, starting with 0, for a duplication without osseous structures, and ending with VIII, for duplication of the tarsal bones. Partial duplication is assigned odd numbers and complete duplication, even numbers. Abbreviations can beusedfor the different associated deformities: “S” for syndactyly, with the number corresponding to the location of webbing; “H” for hypoplastic ray, with the assignment of the affected side (“L” for lateral, and “M” for medial); and “D” for the presence of deviation. 1303

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Fig. 2 The distribution of syndactyly, hypoplastic rays, and deviation by duplication type. 1304

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TABLE V Intrarater and Interrater Agreement

Category Overall Kappa (Range) Agreement Clinician Kappa (Range) Non-Clinician Kappa

Intrarater Duplication 0.79 (0.56-0.87) 82% 0.71 (0.56-0.83) 0.87 Hypoplastic ray 0.75 (0.59-0.93) 85% 0.81 (0.66-0.93) 0.59 Syndactyly 0.59 (0.46-0.78) 69% 0.52 (0.46-0.64) 0.78 Deviation 0.78 (0.49-1.00) 92% 0.71 (0.49-0.84) 1.00 Interrater Duplication 0.72 (0.61-0.83) 88% 0.70 (0.61-0.83) 0.82 Hypoplastic ray 0.54 (0.23-0.76) 82% 0.74 (0.70-0.76) 0.46 Syndactyly 0.48 (0.37-0.78) 79% 0.45 (0.37-0.60) 0.78 Deviation 0.64 (0.53-1.00) 93% 0.62 (0.53-0.76) 0.53

types different from those of our included population were reliability in all 4 categories was lower than that for intrarater suspected in this group. Therefore, selection bias is not likely. reliability. Of the included 43 patients, 16 were male and 27 were female (Table III). Radiographs and clinical photographs were made Discussion between the age of 6 and 18 months in the majority of cases. edial polydactyly of the foot is a rare congenital anom- Medial polydactyly of the foot was seen in 73 feet: 39 right feet Maly 2. The diversity of the anomaly requires individualized and 34 left feet. In most (70%) of the cases, bilateral involve- treatment to diminish shoe-fitting problems and improve ment was noted. aesthetic appearance3,4. In order to plan and evaluate surgical fi All of the feet could be classi ed according to the clas- treatment, the classification of medial polydactyly is useful3. fi si cation system (Table IV). Seven types of duplication were However, current systems are not able to classify all feet and are noted. Duplication type VI was most frequently seen (in 29% of less feasible to use in clinical practice. Therefore, the aim of the the feet). Partial duplication was mostly seen in the distal current study was to develop a reliable and valid classification phalanx (duplication type I; 15%). Twelve feet (16%) showed a system for medial polydactyly of the foot. broad hallux (S0) without the expression of 2 nails. A hypo- A search of the literature resulted in 15 potential cate- = plastic ray was seen in 75% of the feet (medial, n 21; lateral, gories. We included the 4 most mentioned terms, with the = n 34). Deviation of the hallux medially was seen in the ma- exception of ray involvement, because it is plausible to presume jority (73%) of the cases. that these terms are important in the description of medial With respect to duplication in combination with syn- polydactyly. The classification of our own population resulted dactyly, we found that a larger proportion of feet with dupli- in a variety of groups, indicating the diversity of anomalies in cation type II (61%) and type IV (79%) had syndactyly medial polydactyly. However, more specific description of the between 2 halluces (S1) and a larger proportion with dupli- appearance of the feet also revealed frequent combinations. For cation type VI (57%) had syndactyly between a lateral hallux example, syndactyly between lateral and medial halluces was and the second toe (S2) (Fig. 2). Furthermore, duplication type mostly seen with the duplication of the proximal or distal VI never involved a medial hypoplastic ray, whereas phalangeal phalanx, while syndactyly between the lateral hallux and the duplications (type II and type IV) involved a medial hypo- second toe was more frequently seen in metatarsal duplication. plastic ray in the majority of feet. In addition, in all cases with a lateral hypoplastic ray, metatarsal fi The combined classi cation showed a wide variety of duplication was present. Comparable results were also seen in presentations of medial polydactyly. Fourteen foot types were the population of Watanabe et al.14. Moreover, medial hypo- fi scored only once. Larger groups of 1 speci c type were present plastic rays were mostly identified with duplication of the fi in type-IV and type-VI duplications. Eight feet were classi ed proximal and distal phalanges, as was previously reported by fi as IV S1 HM D, and 11 feet were classi ed as VI S2 HL D. Masada et al.13. Group sizes for distal phalangeal (I and II), proximal phalangeal (III and IV), and metatarsal (V and VI) Agreement Analysis duplications were comparable in our population, while for Intrarater agreement and interrater agreement of the Rotter- other populations in the literature3,14, more proximal phalan- dam foot classification are shown in Table V. Classification of geal and metatarsal duplications compared with distal pha- all categories showed moderate to good reliability. Intrarater langeal duplications have been reported. In contrast to the agreement had the lowest k value for syndactyly (k = 0.59), patient populations of Seok et al.12, Belthur et al.3, and Masada with an agreement of 69%. The other 3 categories showed a et al.13, our population more frequently showed an incomplete mean k value that was >0.7. The mean k value for interrater distal phalangeal duplication. Both of these differences in 1305

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findings might be the result of the absence of clear external description of the foot, including the presence of a hypo- appearance of polydactyly and the inability to classify distal plastic ray, deviation, and tarsal duplication. Unfortunately, the phalangeal duplication with their classification systems. In our Watanabe classification system only describes types on the basis study, no feet with type-III and type-VII duplication were pre- of their patient population and does not contain a consistent and sent. However, in thumb polydactyly, these duplication types do uniform description of medial polydactyly14.Furthermore,it exist, and in the study by Watanabe et al., 2 feet with type-III requires drawings for complete description. The Rotterdam foot duplication are described14. This prompted us to include these classification is based on types described in the literature and types in the classification system. All feet in our population could evaluated in our own population. Furthermore, the 4 categories be classified with the proposed system. However, it is known that make specific description easier and more analogous between phenotypic variations are present in different parts of the world; observers. other types of medial polydactyly may be present. By conducting In this study, we developed a new classification system for a review of the literature, we tried to include all described types medial polydactyly of the foot that is based on a review of the in our classification. However, triplications and triphalangism literature and clinical experience. The results of the application are also described in polydactyly 15,18. We did not include these of the classification system to our own population and the categories because of the rarity of these anomalies. Therefore, we agreement analysis showed that the Rotterdam foot classifica- suggest describing rare types in more detail by adding specific tion is a usable system to describe medial polydactyly. There- terms to the classification system. Moreover, if frequently oc- fore, we recommend this system for the description of medial curring categories are found to be missing, the classification polydactyly of the foot. In the future, we hope to demonstrate system allows adaptation. how this classification system can contribute to surgical plan- Classification by all observers was performed with use of ning and evaluation. digital clinical photographs and digital radiographs, the latter also being used in clinical practice. Therefore, duplication level Appendix and hypoplastic rays were assessed during the classification Details of the parameters used in the Embase literature sessions in a manner similar to that of clinical practice. search are available with the online version of this article However, parts of the osseous structures of the foot are as a data supplement at jbjs.org. n 19 not visible on radiographs between 6 and 18 months . Con- NOTE: The authors thank A.J. Pieterse for her contribution to the agreement analysis. sequently, the original duplication level can be different from the observed. Despite this lack of precision, the assessment of duplication level in this study is comparable with assessment of duplication in clinical practice. Syndactyly was assessed using Elise B. Burger, MD1 digital clinical photographs, while assessment in clinical prac- Steven E.R. Hovius, MD, PhD1 Bart J. Burger, MD, PhD2 tice is conducted by physical examination. As syndactyly is not 1 always clearly visible on photographs, this may have led to Christianne A. van Nieuwenhoven, MD, PhD increased uncertainty for the observers compared with the 1Department of Plastic, Reconstructive and Hand Surgery, observation of syndactyly in clinical practice. This could have Erasmus Medical Centre, Rotterdam, the Netherlands resulted in the lower k value found for this category. It is not yet clear whether or not the categories of the 2Department of Orthopedic Surgery, Medical Center Alkmaar, Rotterdam system can effectively be used in the planning of Alkmaar, the Netherlands treatment and the prediction of postoperative functional outcome. E-mail address for E.B. Burger: [email protected] However, Belthur et al. compared the modified Venn-Watson fi fi E-mail address for S.E.R. Hovius: [email protected] classi cation and the more extensive Watanabe classi cation for E-mail address for B.J. Burger: [email protected] 3 surgical planning . They concluded that the comprehensive E-mail address for C.A. van Nieuwenhoven: Watanabe classification is more useful because of the complete [email protected]

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