Skeletal Radiol (1992) 21:371-374
Skeletal Radiology
The metacarpal index - obsolete in M a r f a n syndrome! Keenan N. Joseph, M.B.B.S., M.S. Alyn Hospital, P.O. Box 9117, Jerusalem, Israel
Abstract. 28 patients with definite, probable or p h e n o typic M a r f a n s y n d r o m e u n d e r w e n t m a n u a l and c o m p u terised m e a s u r e m e n t o f their metacarpal indices. Two m e t h o d s were used f r o m a m o n g the several variations q u o t e d in the literature, one involving the average length/width o f metacarpals ~ 5 ( m e t h o d I), and the other dividing the sum o f the lengths by the sum o f the widths o f these metacarpals ( m e t h o d II). The two m e t h o d s are n o t m a t h e m a t i c a l l y identical, and a statistically significant difference was noted between them. Only 50% o f the children in each g r o u p h a d elevated metacarpal indices, m a k i n g it a time-consuming m e t h o d that is indicative only o f the p h e n o t y p e and can be f o u n d in n o r m a l individuals. It should therefore be considered an obsolete m e a s u r e m e n t in M a r f a n syndrome.
Key words: M e t a c a r p a l i n d e x
second to fifth metacarpals by the sum o f their widths at the exact m i d p o i n t o f the shaft. This m e t h o d has since been perpetuated by s t a n d a r d reference w o r k s [5] which use the minimal width o f the metacarpal shaft ( m e t h o d lI). R a n d and associates [9] used m e t h o d II to establish n o r m a l indices in children's hands. Mathematically, m e t h o d s I and II are n o t identical. To date, no a t t e m p t has been m a d e to c o m p a r e the results o b t a i n e d by the two methods. Technological advances, moreover, have yielded equipment that enables determination o f the M C I with greater ease and accuracy t h a n the conventional methods. This paper describes one such machine, a d a p t e d tor m e a s u r e m e n t o f the M C I . C o m p u t e r i z e d measurements using m e t h o d s I and II ( M I and M2 respectively) were c o m p a r e d with the conventional m a n u a l m e t h o d II (M0).
Marfan syndrome
The m e t a c a r p a l index ( M C I ) has been used in a n t h r o p o metry, and in diseases with a r a c h n o d a c t y l y and streblodactyly as a feature [2, 6, 7, 10]. A n elevated M C I was first reported by Sinclair and associates [10] for patients with M a r f a n s y n d r o m e and has been used to c o n f i r m the clinical impression o f a r a c h n o d a c t y l y and to pinpoint potentially affected individuals in a family screening p r o g r a m . The M C I is a fairly simple measurement, but there is a lack o f u n i f o r m i t y in the reported m e t h o d s o f measurement. Sinclair a n d colleagues [10] used the average length-to-width ratios o f the second to fifth metacarpals and m e a s u r e d the widths at the exact m i d p o i n t o f the c o r r e s p o n d i n g m e t a c a r p a l shaft. Parish [6, 7] modified this m e t h o d using the minimal width o f the m e t a c a r p a l shaft ( m e t h o d I) to calculate the index and established values for the diagnosis o f M a r f a n s y n d r o m e o f 8.4 in males a n d 9.2 in females. Joseph a n d M e a d o w [4] established n o r m a l figures for infants up to the age o f 2 years but erroneously q u o t e d Sinclair and co-workers' m e t h o d as involving a division o f the sum o f the lengths o f the
Materials and methods A Numonics model 1224 electronic digitizer (Digital Equipment Corporation, Maynard, Mass.) was used at the Alfred I. DuPont Institute, Wilmington, Delaware, for the MCI measurements. This digitizer is suited for use with radiographs or other translucent material because the cursor arm can be placed over a lighted table unlike other digitizers, which require an opaque pad. There are four major components to each digitizer. A display console (Fig. I A) contains the display tubes and houses most of the electronic circuits for the translation, calculation, and demonstration of data. A reading head (Fig. 1 B) contains the carriage and the optical pick-up system for detecting carriage translation along the x-axis (abscissa). A magnifying cursor is inserted through one end of a transverse arm that is moved by the operator along the y-axis (ordinate). The keyboard (Fig. 1 C) has 16 character keys and 4 shift keys and, once initialized, controls the operation of the unit. This digitizer can measure lengths and widths and also transmit these measurements to programmable calculators and printouts. Data input was by way of an RS232 interface to the DBC LSI II computer (Fig. l D). This computer consists of a DBS LSI LV/ 213 computer bytes memory, 2 RLOI discs containing five million characters each, I UT ]05 CRT terminal, 1 LA 120 printing terminal, and an RTLKL operating system. The unit employs Fortran IV language. 9 1992 International Skeletal Society
372
K.N. Joseph: Metacarpal index
Fig. 1 A-D. The digitiser setup comprising the display console (A) reading head (B), keyboard (C), and computer (D)
A total of 28 patients from the A.I. DuPont Institute, Wilmington, Delaware, with the provisional diagnosis of Marfan syndrome formed the basis of this study. There were 13 males and 15 females, with an average age of 16.4 years (range 2-28 years). Each patient underwent evaluation by an orthopedic surgeon, pediatrician, ophthalmologist, and pediatric cardiologist. The diagnostic work-up included electrocardiography and echocardiography, slit lamp and funduseopic examination, and screening for homocystinuria by urine nitroprusside testing. The patients were divided into three groups, definite, probable and phenotypic Marfan in the recently proposed classification [3]. Anteroposterior radiographs of the left hand with a film-focus distance of 100 em were used to determine the MCI manually using method II (division of the sum of lengths by the sum of widths of metacarpals 2, 3, 4, and 5) and with the digitizer using the above method and method I (average of the lengths/widths of metacarpals 2-5)_ The measurements were all done by 1 person in two sessions at the end of the study to eliminate observer variation. The diagnostic values established by Parish for the MCI in Marfan syndrome (8.4 in males and 9.2 in females) were used in this study. The paired t-test and the Wileoxon matched pairs test were used for statistical analysis.
Results Only 5 o f the 12 patients (42%) with definite M a r f a n syndrome, 5 o f the 9 (56%) in the p r o b a b l e group, and 4 o f the 7 in the p h e n o t y p i c g r o u p (57%) h a d an elevated M C I , indicating radiologically verifiable a r a c h n o d a c t y l y (Table 1). N o statistical difference was noted with age. The greatest differences between M 0 and M1 were + 0 . 6 5 and - 0 . 5 3 , between M0 and M 2 were + 0 . 6 9 and - 0 . 5 1 , and between M I and M 2 were 0.18 and - 0 . 0 3 . The differences between the m a n u a l and semiaut o m a t e d m e t h o d s were therefore greater t h a n those between the computerised methods. N o significant difference was f o u n d between the means o f M0, M I , and M2. Significant difference was noted between M1 and M2 using the W i l c o x o n m a t c h e d pairs test in the p r o b able and p h e n o t y p i c M a r f a n s y n d r o m e groups, where the M1 m e a s u r e m e n t was consistently larger than M2. Analysis o f variance between the different m e t h o d s in the three groups was w i t h o u t significance.
K.N. Joseph: Metacarpal index
373
Table 1. Results of metacarpal index measurements No.
Age
Sex
(years)
Manual
Computerized
Differences
MI
M2
M0-M1
MI-M2
M0-M2
9.20 12.17 9.69 7.03 6.63 8.21 9.14 7.87 9.73 8.48 7.85 9.20
9.23 12.09 9.62 7.05 6.61 8.23 9.10 7.87 9.65 8.49 7.83 9.17
0.10 -0.47 --0.19 0.07 --0.53 0.29 --0.04 0.13 0.47 0.22 0.35 --0.20
--0.03 0.08 0.07 --0.02 0.02 --0.02 0.04 0 0.08 --0.01 0.02 0.03
0.07 --0.39 --0.12 --0.05 -0.51 --0.27 0 0.13 0.55 0.21 0.37 --0.17
8.77
8.75
0.02 NS
0.02 NS
0.04 NS
9.95 7.80 8.50 8.46 9.71 8.55 9.20 7.65 8.71
9.91 7.79 8.42 8.43
9.66
0.65 0.10 0.60 -- 0.26 --0.21 --0.15 0.20 0.05 --0.01
0.04 0.01 0.08 0.03 0_05 0.01 0.10 0_02 0.i1
0.69 0.11 0.68 -- 0.23 --0.16 --0.14 0.30 0.07 0.I0
8.73
8.68
0.11 NS
0.05 NS
0.16 NS
10.20 9.18 9.54 8.26 6.67 7.25 7.66
10.21 9.10 9.49 8.25 6.67 7.07 7.66
--0.20 0.02 0.46 0.44 0.23 0.45 0.14
--0.01 0.08 0.05 0.01 0.0 0.18 0.0
--0.21 0.1 0.51 0.45 0.23 0,63 0.14
8.39
8.35
0.22 NS
0.04 NS
0.26 NS
M0
Definite Matfan 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
11 20 15 13 13 15 23 5 18 17 19 27
M M F M M F M M F F F F
Average t-test
9.3 11.7 9.5 7.1 6.1 8.5 9.1 8.0 10.2 8.7 8.2 9.0 8.78
Probable Marfan 1. 2. 3. 4. 5. 6. 7. 8. 9.
16 17 23 14 12 22 18 14 18
M M F F F M F F M
Average t-test
10.6 7.9 9.1 8_2 9_5 8.4 9.4 7.7 8.7 8.83
8.54 9_10 7_63 8.60
Phenotypic Marfan 1. 2. 3. 4. 5. 6. 7.
Average t-test
19 20 10 13 28 2 14
F M F M M F F
10.0 9.2 10 8.7 6.9 7.7 7.8 8.61
M0, Manual measurement; M1, computerized measurement, m e t h o d I; M2, computerized nleasurement, method II
Discussion The lack of uniformity in the methods of calculation of the MCI is surprising and has not been commented on beforc. The different indices quoted by various reference works also lack uniformity as they employ different variations of the quoted methods. The most slender portion of the metacarpal shaft is more proximal to the exact midpoint of the metacarpal shaft. Mathematically as well, division of the sum of lengths by the sum of widths of the second to fifth metacarpals (M2) is not identical with the average of the length/width ratios of these metacarpals (M1)i.e., ~ L / ~ B . ~ ( L / B ) / 4 . M2 is a simpler calculation because only one division is involved. However, this method tends to provide a more uniform result mathematically, while MI is more sensi-
tive to unusual variations in the sizes of the metacarpals. Even though the mean difference between the various techniques was small and the range of differences was also small compared with the between patient variability, there are significant differences between the two methods. This raises the question of the validity of MCIs quoted by articles when different equations were used. The digitizer improves speed and accuracy in the measurement of the MCI and allows for relatively simple training of the necessary personnel. It can be adapted to record hand pattern profiles or to measure spinal canal widths. Sinclair and colleagues [10] reported on the MCI of 20 patients with Marfan syndrome and compared them with 100 randomly selected normal patients; they noted a significant difference in the two groups that appeared
374
to increase with age. They suggested that Marfan syndrome should be considered whenever this objective measurement of arachnodactyly was accompanied by an arm span exceeding the height by more than 7.5 cm and a lower segment measurement of at least 5 cm more than the upper segment. It has now become standard practice for these measurements of the trunk and hands to be made in a patient thought to have Marfan syndrome. Some 50% of the patients in each of our groups had elevated indices. That half of the definite Marfan patients had raised indices would indicate the measurements as having some value. However, these were preselected patients in that they were all provisionally diagnosed as Marfan patients after a preliminary examination in the orthopaedic outpatient clinics of a tertiary referral center. Seen in this context, the fact that 57% of the phenotypic group (in whom Marfan syndrome was ruled out due to the absence of major ophthalmological or cardiological signs) had elevated indices has more significance, making it more indicative of the phenotype than diagnostic of Marfan syndrome. It has to be remembered that inheritance in Marfan syndrome follows the autosomal dominant pattern with incomplete penetrance of the gene leading to a variable phenotype. Also, many of the individual findings of this syndrome are also encountered in normal individuals. The diagnosis of Marfan syndrome, moreover, has undergone an evolution from previously being made purely on clinical grounds [1] to currently being dependent on sophisticated tests such as echocardiograms and slit lamp ophthalmological examinations after full dilatation of the pupil [8]. While weighing up the clinical and laboratory data in this series, it was noted that the MCI had no influence whatsoever on the diagnostic decision-making, discounting its value as even a "minor" sign. Simple clinical signs, originally claimed to be specific for Marfan syndrome, are currently used only for confirmation of a clinical suspicion [1/, 12]. They are not diagnostic of Marfan syndrome but help to de-
K.N. Joseph: Metacarpal index
cide whether further investigations are called for. They have the virtue of simplicity and can be performed within a minute. The MCI, in contrast, is a time-consuming, radiation-cumulative, nonstandardised, nonspecific measurement that currently has an insignificant role to play in the diagnostic workup of Marfan syndrome. It should, therefore, be considered obsolete here. Acknowledgements. I would like to thank J. Richard Bowen, M.D., for permission to publish this article, Joe McCullock who programmed the computer. Sara Schutzman who assisted with the radiographic measurement, and Raya Steinberg who helped with the statistical analysis. References 1. Brenton DP, Dow CJ, James JIP, Hay RL, Wynne-Davies R (1972) Homocystinuria and Marfan's syndrome: a comparison, J Bone Joint Surg [Br] 54:277 2. Eldridge R (1964) The metacarpal index: a useful aid in the diagnosis of the Marfan syndrome. Arch Intern Med 113 : 140 3. Joseph KN, Kane HA, Milner RS, Steg NL, Williamson MB, Bowen JR (1992) Orthopaedic aspects of the Marfan phenotype. Clin Orthop 277:251 4. Joseph MC, Meadow SR (1969) The metacarpal index of infants. Arch Dis Child 44:515 5. Lusted LB, Keats TE (1978) Atlas of roentgenographic measurement, 4th edn. Year Book Medical, Chicago, p 131 6. Parish JG (1966) Radiographic measurements of the skeletal structure of the normal hand. Br J Radiol 39: 52 7. Parish JG (1967) Skeletal hand charts in inherited connective tissue disease. J Med Genet 4:227 8. Pyeritz RE, McKusick VA (1979) The Marfan syndrome: diagnosis and management. N Engl J Med 300 : 772 9. Rand TC, Edwards DK, Bay CA, Jones KL (1980) The metacarpal index in normal children. Pediatr Radiol 9:31 10. Sinclair RJG, Kitchin AH, Turner RWD (1960) The Marfan syndrome. J Med 113:19 11. Steinberg I (1968) A simple screening test for the Marfan syndrome. Am J Roentgenol Radium Ther Nucl Med 97 : 118 12. Walker DR, Ditkoff TJ, Hen'ing JA, Mauldin DM (1970) A useful physical finding in the Marfan syndrome. Arch Intern Med 126:276
Skeletal Radiology
The metacarpal index - obsolete in M a r f a n syndrome! Keenan N. Joseph, M.B.B.S., M.S. Alyn Hospital, P.O. Box 9117, Jerusalem, Israel
Abstract. 28 patients with definite, probable or p h e n o typic M a r f a n s y n d r o m e u n d e r w e n t m a n u a l and c o m p u terised m e a s u r e m e n t o f their metacarpal indices. Two m e t h o d s were used f r o m a m o n g the several variations q u o t e d in the literature, one involving the average length/width o f metacarpals ~ 5 ( m e t h o d I), and the other dividing the sum o f the lengths by the sum o f the widths o f these metacarpals ( m e t h o d II). The two m e t h o d s are n o t m a t h e m a t i c a l l y identical, and a statistically significant difference was noted between them. Only 50% o f the children in each g r o u p h a d elevated metacarpal indices, m a k i n g it a time-consuming m e t h o d that is indicative only o f the p h e n o t y p e and can be f o u n d in n o r m a l individuals. It should therefore be considered an obsolete m e a s u r e m e n t in M a r f a n syndrome.
Key words: M e t a c a r p a l i n d e x
second to fifth metacarpals by the sum o f their widths at the exact m i d p o i n t o f the shaft. This m e t h o d has since been perpetuated by s t a n d a r d reference w o r k s [5] which use the minimal width o f the metacarpal shaft ( m e t h o d lI). R a n d and associates [9] used m e t h o d II to establish n o r m a l indices in children's hands. Mathematically, m e t h o d s I and II are n o t identical. To date, no a t t e m p t has been m a d e to c o m p a r e the results o b t a i n e d by the two methods. Technological advances, moreover, have yielded equipment that enables determination o f the M C I with greater ease and accuracy t h a n the conventional methods. This paper describes one such machine, a d a p t e d tor m e a s u r e m e n t o f the M C I . C o m p u t e r i z e d measurements using m e t h o d s I and II ( M I and M2 respectively) were c o m p a r e d with the conventional m a n u a l m e t h o d II (M0).
Marfan syndrome
The m e t a c a r p a l index ( M C I ) has been used in a n t h r o p o metry, and in diseases with a r a c h n o d a c t y l y and streblodactyly as a feature [2, 6, 7, 10]. A n elevated M C I was first reported by Sinclair and associates [10] for patients with M a r f a n s y n d r o m e and has been used to c o n f i r m the clinical impression o f a r a c h n o d a c t y l y and to pinpoint potentially affected individuals in a family screening p r o g r a m . The M C I is a fairly simple measurement, but there is a lack o f u n i f o r m i t y in the reported m e t h o d s o f measurement. Sinclair a n d colleagues [10] used the average length-to-width ratios o f the second to fifth metacarpals and m e a s u r e d the widths at the exact m i d p o i n t o f the c o r r e s p o n d i n g m e t a c a r p a l shaft. Parish [6, 7] modified this m e t h o d using the minimal width o f the m e t a c a r p a l shaft ( m e t h o d I) to calculate the index and established values for the diagnosis o f M a r f a n s y n d r o m e o f 8.4 in males a n d 9.2 in females. Joseph a n d M e a d o w [4] established n o r m a l figures for infants up to the age o f 2 years but erroneously q u o t e d Sinclair and co-workers' m e t h o d as involving a division o f the sum o f the lengths o f the
Materials and methods A Numonics model 1224 electronic digitizer (Digital Equipment Corporation, Maynard, Mass.) was used at the Alfred I. DuPont Institute, Wilmington, Delaware, for the MCI measurements. This digitizer is suited for use with radiographs or other translucent material because the cursor arm can be placed over a lighted table unlike other digitizers, which require an opaque pad. There are four major components to each digitizer. A display console (Fig. I A) contains the display tubes and houses most of the electronic circuits for the translation, calculation, and demonstration of data. A reading head (Fig. 1 B) contains the carriage and the optical pick-up system for detecting carriage translation along the x-axis (abscissa). A magnifying cursor is inserted through one end of a transverse arm that is moved by the operator along the y-axis (ordinate). The keyboard (Fig. 1 C) has 16 character keys and 4 shift keys and, once initialized, controls the operation of the unit. This digitizer can measure lengths and widths and also transmit these measurements to programmable calculators and printouts. Data input was by way of an RS232 interface to the DBC LSI II computer (Fig. l D). This computer consists of a DBS LSI LV/ 213 computer bytes memory, 2 RLOI discs containing five million characters each, I UT ]05 CRT terminal, 1 LA 120 printing terminal, and an RTLKL operating system. The unit employs Fortran IV language. 9 1992 International Skeletal Society
372
K.N. Joseph: Metacarpal index
Fig. 1 A-D. The digitiser setup comprising the display console (A) reading head (B), keyboard (C), and computer (D)
A total of 28 patients from the A.I. DuPont Institute, Wilmington, Delaware, with the provisional diagnosis of Marfan syndrome formed the basis of this study. There were 13 males and 15 females, with an average age of 16.4 years (range 2-28 years). Each patient underwent evaluation by an orthopedic surgeon, pediatrician, ophthalmologist, and pediatric cardiologist. The diagnostic work-up included electrocardiography and echocardiography, slit lamp and funduseopic examination, and screening for homocystinuria by urine nitroprusside testing. The patients were divided into three groups, definite, probable and phenotypic Marfan in the recently proposed classification [3]. Anteroposterior radiographs of the left hand with a film-focus distance of 100 em were used to determine the MCI manually using method II (division of the sum of lengths by the sum of widths of metacarpals 2, 3, 4, and 5) and with the digitizer using the above method and method I (average of the lengths/widths of metacarpals 2-5)_ The measurements were all done by 1 person in two sessions at the end of the study to eliminate observer variation. The diagnostic values established by Parish for the MCI in Marfan syndrome (8.4 in males and 9.2 in females) were used in this study. The paired t-test and the Wileoxon matched pairs test were used for statistical analysis.
Results Only 5 o f the 12 patients (42%) with definite M a r f a n syndrome, 5 o f the 9 (56%) in the p r o b a b l e group, and 4 o f the 7 in the p h e n o t y p i c g r o u p (57%) h a d an elevated M C I , indicating radiologically verifiable a r a c h n o d a c t y l y (Table 1). N o statistical difference was noted with age. The greatest differences between M 0 and M1 were + 0 . 6 5 and - 0 . 5 3 , between M0 and M 2 were + 0 . 6 9 and - 0 . 5 1 , and between M I and M 2 were 0.18 and - 0 . 0 3 . The differences between the m a n u a l and semiaut o m a t e d m e t h o d s were therefore greater t h a n those between the computerised methods. N o significant difference was f o u n d between the means o f M0, M I , and M2. Significant difference was noted between M1 and M2 using the W i l c o x o n m a t c h e d pairs test in the p r o b able and p h e n o t y p i c M a r f a n s y n d r o m e groups, where the M1 m e a s u r e m e n t was consistently larger than M2. Analysis o f variance between the different m e t h o d s in the three groups was w i t h o u t significance.
K.N. Joseph: Metacarpal index
373
Table 1. Results of metacarpal index measurements No.
Age
Sex
(years)
Manual
Computerized
Differences
MI
M2
M0-M1
MI-M2
M0-M2
9.20 12.17 9.69 7.03 6.63 8.21 9.14 7.87 9.73 8.48 7.85 9.20
9.23 12.09 9.62 7.05 6.61 8.23 9.10 7.87 9.65 8.49 7.83 9.17
0.10 -0.47 --0.19 0.07 --0.53 0.29 --0.04 0.13 0.47 0.22 0.35 --0.20
--0.03 0.08 0.07 --0.02 0.02 --0.02 0.04 0 0.08 --0.01 0.02 0.03
0.07 --0.39 --0.12 --0.05 -0.51 --0.27 0 0.13 0.55 0.21 0.37 --0.17
8.77
8.75
0.02 NS
0.02 NS
0.04 NS
9.95 7.80 8.50 8.46 9.71 8.55 9.20 7.65 8.71
9.91 7.79 8.42 8.43
9.66
0.65 0.10 0.60 -- 0.26 --0.21 --0.15 0.20 0.05 --0.01
0.04 0.01 0.08 0.03 0_05 0.01 0.10 0_02 0.i1
0.69 0.11 0.68 -- 0.23 --0.16 --0.14 0.30 0.07 0.I0
8.73
8.68
0.11 NS
0.05 NS
0.16 NS
10.20 9.18 9.54 8.26 6.67 7.25 7.66
10.21 9.10 9.49 8.25 6.67 7.07 7.66
--0.20 0.02 0.46 0.44 0.23 0.45 0.14
--0.01 0.08 0.05 0.01 0.0 0.18 0.0
--0.21 0.1 0.51 0.45 0.23 0,63 0.14
8.39
8.35
0.22 NS
0.04 NS
0.26 NS
M0
Definite Matfan 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
11 20 15 13 13 15 23 5 18 17 19 27
M M F M M F M M F F F F
Average t-test
9.3 11.7 9.5 7.1 6.1 8.5 9.1 8.0 10.2 8.7 8.2 9.0 8.78
Probable Marfan 1. 2. 3. 4. 5. 6. 7. 8. 9.
16 17 23 14 12 22 18 14 18
M M F F F M F F M
Average t-test
10.6 7.9 9.1 8_2 9_5 8.4 9.4 7.7 8.7 8.83
8.54 9_10 7_63 8.60
Phenotypic Marfan 1. 2. 3. 4. 5. 6. 7.
Average t-test
19 20 10 13 28 2 14
F M F M M F F
10.0 9.2 10 8.7 6.9 7.7 7.8 8.61
M0, Manual measurement; M1, computerized measurement, m e t h o d I; M2, computerized nleasurement, method II
Discussion The lack of uniformity in the methods of calculation of the MCI is surprising and has not been commented on beforc. The different indices quoted by various reference works also lack uniformity as they employ different variations of the quoted methods. The most slender portion of the metacarpal shaft is more proximal to the exact midpoint of the metacarpal shaft. Mathematically as well, division of the sum of lengths by the sum of widths of the second to fifth metacarpals (M2) is not identical with the average of the length/width ratios of these metacarpals (M1)i.e., ~ L / ~ B . ~ ( L / B ) / 4 . M2 is a simpler calculation because only one division is involved. However, this method tends to provide a more uniform result mathematically, while MI is more sensi-
tive to unusual variations in the sizes of the metacarpals. Even though the mean difference between the various techniques was small and the range of differences was also small compared with the between patient variability, there are significant differences between the two methods. This raises the question of the validity of MCIs quoted by articles when different equations were used. The digitizer improves speed and accuracy in the measurement of the MCI and allows for relatively simple training of the necessary personnel. It can be adapted to record hand pattern profiles or to measure spinal canal widths. Sinclair and colleagues [10] reported on the MCI of 20 patients with Marfan syndrome and compared them with 100 randomly selected normal patients; they noted a significant difference in the two groups that appeared
374
to increase with age. They suggested that Marfan syndrome should be considered whenever this objective measurement of arachnodactyly was accompanied by an arm span exceeding the height by more than 7.5 cm and a lower segment measurement of at least 5 cm more than the upper segment. It has now become standard practice for these measurements of the trunk and hands to be made in a patient thought to have Marfan syndrome. Some 50% of the patients in each of our groups had elevated indices. That half of the definite Marfan patients had raised indices would indicate the measurements as having some value. However, these were preselected patients in that they were all provisionally diagnosed as Marfan patients after a preliminary examination in the orthopaedic outpatient clinics of a tertiary referral center. Seen in this context, the fact that 57% of the phenotypic group (in whom Marfan syndrome was ruled out due to the absence of major ophthalmological or cardiological signs) had elevated indices has more significance, making it more indicative of the phenotype than diagnostic of Marfan syndrome. It has to be remembered that inheritance in Marfan syndrome follows the autosomal dominant pattern with incomplete penetrance of the gene leading to a variable phenotype. Also, many of the individual findings of this syndrome are also encountered in normal individuals. The diagnosis of Marfan syndrome, moreover, has undergone an evolution from previously being made purely on clinical grounds [1] to currently being dependent on sophisticated tests such as echocardiograms and slit lamp ophthalmological examinations after full dilatation of the pupil [8]. While weighing up the clinical and laboratory data in this series, it was noted that the MCI had no influence whatsoever on the diagnostic decision-making, discounting its value as even a "minor" sign. Simple clinical signs, originally claimed to be specific for Marfan syndrome, are currently used only for confirmation of a clinical suspicion [1/, 12]. They are not diagnostic of Marfan syndrome but help to de-
K.N. Joseph: Metacarpal index
cide whether further investigations are called for. They have the virtue of simplicity and can be performed within a minute. The MCI, in contrast, is a time-consuming, radiation-cumulative, nonstandardised, nonspecific measurement that currently has an insignificant role to play in the diagnostic workup of Marfan syndrome. It should, therefore, be considered obsolete here. Acknowledgements. I would like to thank J. Richard Bowen, M.D., for permission to publish this article, Joe McCullock who programmed the computer. Sara Schutzman who assisted with the radiographic measurement, and Raya Steinberg who helped with the statistical analysis. References 1. Brenton DP, Dow CJ, James JIP, Hay RL, Wynne-Davies R (1972) Homocystinuria and Marfan's syndrome: a comparison, J Bone Joint Surg [Br] 54:277 2. Eldridge R (1964) The metacarpal index: a useful aid in the diagnosis of the Marfan syndrome. Arch Intern Med 113 : 140 3. Joseph KN, Kane HA, Milner RS, Steg NL, Williamson MB, Bowen JR (1992) Orthopaedic aspects of the Marfan phenotype. Clin Orthop 277:251 4. Joseph MC, Meadow SR (1969) The metacarpal index of infants. Arch Dis Child 44:515 5. Lusted LB, Keats TE (1978) Atlas of roentgenographic measurement, 4th edn. Year Book Medical, Chicago, p 131 6. Parish JG (1966) Radiographic measurements of the skeletal structure of the normal hand. Br J Radiol 39: 52 7. Parish JG (1967) Skeletal hand charts in inherited connective tissue disease. J Med Genet 4:227 8. Pyeritz RE, McKusick VA (1979) The Marfan syndrome: diagnosis and management. N Engl J Med 300 : 772 9. Rand TC, Edwards DK, Bay CA, Jones KL (1980) The metacarpal index in normal children. Pediatr Radiol 9:31 10. Sinclair RJG, Kitchin AH, Turner RWD (1960) The Marfan syndrome. J Med 113:19 11. Steinberg I (1968) A simple screening test for the Marfan syndrome. Am J Roentgenol Radium Ther Nucl Med 97 : 118 12. Walker DR, Ditkoff TJ, Hen'ing JA, Mauldin DM (1970) A useful physical finding in the Marfan syndrome. Arch Intern Med 126:276
