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Diversity of proximal femoral medullary canal
The Journal of Arthroplasty Vol. 15 No. 1 2000
Diversity of Proximal Femoral Medullary Canal H.-J. L a i n e , M D , M. U. K. L e h t o , M D , P h D , a n d T. M o i l a n e n , M D , P h D
A b s t r a c t : To analyze the endosteal dimensions of the proximal femoral medullary canal, we studied 50 cadaver femora using computed tomography and a border detection method from 20 mm above tile lesser trochanter, the osteotomy level, down to the isthmus. We documented the presence o1 a dense calcar septum in 96% of femora studied. In addition to mediolateral, anteroposterior, and neck-oriented dimensions, we calculated canal flare indices (CFIs) between the osteotomy level and the isthmus and metaphyseal canal flare indices (MCFIs) between tile osteotomy level and the level 20 m m below the lesser trochanter midpoint to describe the shape of the endosteal cavity. With respect to the canal opening, the anteroposterior and mediolateral planes parallelled each other over the entire region as indicated by the high correlation ( r = . 7 , P < . 0 0 1 ) between the CFIs in these directions. The prediction of one dimension from another was unreliable in the metaphyseal region, where bone ingrowth is supposed to occur in a femoral prosthesis. The MCFI seemed to be the best parameter to distinguish the various types of proximal femoral canal shapes. We have described the variability of the proximal femoral endosteal dimensions in detail and find that the wide variation in the shape and size of the proximal femoral medullary canal means that it is almost impossible to achieve 100% cortical contact with the stem, especially in the metaphysis. K e y w o r d s : hip, arthroplagty, femoral anatomy, medullary canal, endoprosthesis design.
Good clinical results and midterm survival rates of cementless total hip arthroplasties have been published [1,2], although failures of endoprosthetic design still occur, even in the short-term [3,4]. Cemented arthroplasty offers excellent long-term survival rates [5], but problems are still encountered with young patients [6,7]. Thus, improvements in cementless hip arthroplasty aim at durable results even in younger, physically active patients. Femoral stern micromotion studies have emphasized the importance of metaphyseal endosteal stem fit in the reduction of torsional motion [8-10], enabling bony ingrowth and thus stable biologic
fixation [ 11-13]. Finite-element studies [ 14,15] and photoelastic coating strain analysis [16] have also demonstrated the importance of metaphyseal fit in achieving physiologic-like implant-bone load transfer and in minimizing stress shielding and disadvantageous bone remodeling. Modular [17,18] and custom-made [19-21] femoral prostheses have been designed to optimizefit andfill, to minimize micromotion, and thus to maximize bone ingrowth. Early radiologic and clinical results have been promising, but problems have been encountered (eg, intraoperative femoral fractures [20,22,23]), and disappointing results of Cementless, nonporous custommade femoral components have been reported [24]. Noble et al. [25] demonstrated that there is no universal shape for the femoral cavity. They described great variability of endosteal anatomy and poor predictability of endosteal dimensions by using standard radiographs of 200 cadaver femora. Rubin et al. [26] used computed tomography (CT) combined with a border detection computer program to
From the Division of Orthopaedics, Department of Surgery, Tampere University Hospital and Medical School, University of Tampere. Tampere, Finland. Submitted October 29, 1997; accepted November 30, 1998. Reprint requests: H.-J. Laine, MD, Tampere University Hospital, Department of Surgery, PO Box 2000, SF-33521 Tampere, Finland. C6pyrighl © 2000 by Churchill Livingstone® 0883- 5403/00/1501-0014510.OO/O
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Diversity of Proximal Femoral Medullary Canal
study femoral m o r p h o l o g y and reported similar results. H u s m a n n et al. [27] described the endosteal m o r p h o l o g y of the proximal f e m u r focusing on the variation of the m e t a p h y s e a l and calcar region. It seems clear that a single cementless stem design, even if several sizes are available, does not fit all femoral cavities [25,27]. The purpose of the present study was to analyze the dimensions m e a s u r e d from the endosteal surface of the cadaver proximal femoral m e d u l l a r y canal in the diaphysis and in the m e t a p h y s i s to d e t e r m i n e w h e t h e r it is rational to aim at perfect fit and fill in cementless femoral c o m p o n e n t fixation or if the variability of the inner surface is so great that 100% cortical contact cannot in practice be achieved.
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tal 2 - m m - t h i c k slices in 5 - m m intervals u p w a r d from 20 m m below the zero-level and 14 4 - m m thick slices in 10-ram intervals d o w n w a r d from 20 m m b e l o w the zero-level w e r e taken. The endosteal surface was detected from the CT data by a borderdetection computer program, which was developed especially for this purpose as previously described [28]. The femoral cavity was divided into 3 structurally different parts. The diaphyseal bone of the femoral shaft has a clear border b e t w e e n the cortical and cancellous bone. The lesser t r o c h a n t e r area has posteromedially a calcar s e p t u m of nearly cortical density dividing the femoral cavity from the lesser trochanter. Above the lesser trochanter, the metaphyseal trabecular b o n e transforms gradually to cortical bone, especially on the lateral side. The detectable lateral cortex normally extends 20 to 40 m m above the lesser t r o c h a n t e r midpoint [28]. The longest mediolateral, anteroposterior, and oblique endosteal diameters of the slices w e r e computed. The level of the slice (a value given as the distance below the zero-level) that had the n a r r o w est mediolateral d i a m e t e r (ie, the isthmus width) was defined as the isthmus level. At the o s t e o t o m y level (20 m m above T), the endosteal neck anteversion was defined as the angle b e t w e e n the longest oblique (ie, the neck oriented) d i a m e t e r and the horizontal mediolateral d i a m e t e r (Fig. 1). The pres-
Materials and Methods CT was p e r f o r m e d in 50 cadaver femora (21 left and 29 right, from separate white individuals, 36 male and 14 female, average age at death 70 years [range, 38-87 years] ). Institutional Ethical C o m m i t tee approval was obtained for the use of cadaver femora. Two femora w e r e placed on thermoplastic (Plexiglas) at the same time so that their vertical axes were parallel to the table axis and their rotations were neutral. They w e r e placed with their lesser t r o c h a n t e r midpoints at the same vertical level (referred to as the zero-level, T). Sixteen horizon-
Ac T level E
T-20
H G ~
level
I
isthmus lever
J
A B C 0 E F G H i
= = = = = = = = =
the the the the the the the the the
longest longest longest longest longest longest longest longest longest
med~oloteral diometer, T+20 level onterioposterlor d;orneter, T+20 level obl;que diometer, T+20 level medlo$oterol diometer. T - 2 0 level onterloposterlor diometer. T - 2 0 level oblique diometer. "1"-20 level medloloterol diometer, isthmus level onter~oposterlot diometer, isthmus level oblique diometer, ; s t h m u s level
G = medloloteral conol (lore index. CFI H = onterioposterior conol (Io~e index. CFIop G = collum oriented Conol flore index. CFIco
A ~/ ~ = medioloterol proximol conol flore index, PCFI B onterloposter;or proximol conol flore index. PCFtop C // O = collum oriented prox;mol t o n a l (lore index, PCFI¢o
Fig. 1. The longest mediolateral, anteroposterion and oblique diameters were computed for each slice. Canal flare indices (CFIs) were calculated for each femur by the diameters of the isthmus level slice and the osteotomy level slice (20 mm above lesser trochanter midpoint, T 4. 20). Metaphyseal canal flare indices (MCFIs) were calculated for each femur by the diameters of the slice of 20 mm below ( T - 20) the lesser trochanter midpoint and the osteotomy level slice (T 4- 20). Endosteal neck anteversion angle (e0 was measured from the osteotomy level slice of each femur.
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e n c e of a d e n s e c o r t i c a l - l i k e s e p t u m in t h e c a l c a r f e m o r a l a r e a w a s also s t u d i e d . Canal flare indices (CFIs) w e r e c a l c u l a t e d as t h e ratio b e t w e e n t h e m e d i o l a t e r a l c a n a l w i d t h at t h e o s t e o t o m y level a n d t h e i s t h m u s w i d t h [25]. Anteroposterior CFIs w e r e c a l c u l a t e d s i m i l a r l y f r o m t h e a n t e r o p o s t e r i o r d i m e n s i o n s . Neck-oriented CFIs w e r e d e t e r m i n e d as t h e ratio b e t w e e n t h e l o n g e s t o b l i q u e d i m e n s i o n at t h e o s t e o t o m y level a n d t h e i s t h m u s w i d t h (Fig. 1). T h e s e figures w e r e d e f i n e d to d e scribe t h e f e m o r a l c a v i t y o p e n i n g f r o m t h e i s t h m u s to t h e o s t e o t o m y level. To specify t h e v a r i a b i l i t y of t h e f e m o r a l m e t a p h y seal o p e n i n g , t h e metaphyseal canalflare index (MCFI) w a s d e t e r m i n e d (ie, t h e ratio b e t w e e n m e d i o l a t e r a l c a n a l w i d t h 20 m m a b o v e a n d 20 m m b e l o w t h e lesser t r o c h a n t e r m i d p o i n t IT] ). Anteroposterior MCFIs a n d t h e neck-oriented MCFIs w e r e d e t e r m i n e d in a s i m i l a r f a s h i o n (Fig. 1). To a n a l y z e t h e r e g u l a r i t y of t h e s h a p e of t h e m e t a p h y s e a l f e m o r a l cavity, w e c a l c u l a t e d t h e b i v a r i a t e c o r r e l a t i o n s of t h e d i m e n sions in p r o x i m a l slices (from 20 m m b e l o w to 20 m m a b o v e T). To s t u d y t h e r a t e of o p e n i n g f r o m d i a p h y s i s to m e t a p h y s i s , w e d e t e r m i n e d t h e e n l a r g e m e n t r a t e of t h e f e m o r a l c a v i t y (ie, t h e i n c r e a s e of t h e m e d i o l a t eral d i a m e t e r p e r 1 0 - m m slice i n t e r v a l ) . D e s c r i p t i v e statistics, d i s t r i b u t i o n graphics, a n d P e a r s o n l i n e a r c o r r e l a t i o n s w i t h s c a t t e r p l o t s w e r e u s e d to d e t e r m i n e t h e typical f e m o r a l e n d o s t e a l d i m e n s i o n s a n d their variation.
T a b l e 1. Proximal Femoral Endosteal D i m e n s i o n s (ram) at Different Levels (n = 50) Mediolateral Dimensions
Anteroposterior Dimensions
Longes! Oblique Dimensions
Level
Mean
SD
Mean
SD
Mean
SD
T + 20 T + 15 T + 10 T+ 5 T T - 10 T - 20 T - 30 T - 40 T - 60 Isthmus T - 150
45.42 40.43 35.49 31.51 28.73 25.03 20.41 17.74 15.73 13.43 11.06 12.65
4.46 4.59 4.23 4.05 3.21 2.58 2.14 1.93 1.97 2.12 1.88 2.03
31.39 29.79 28.55 26.95 25.58 23.85 20.71 18.27 16.77 15.92 14.09 14.82
3.45 3.18 3.06 2.95 2.86 2.32 2.47 2.39 2.56 2.67 2.81 2.64
47.09 41.95 37.53 33.95 31.01 27.58 23.19 20.08 18.68 17.33 14.79 15.59
4.98 4.76 4.31 4.22 3.65 2.53 2.74 2.52 2.51 2.65 2.96 2.69
SD, standard deviation; T, trochanter.
w a s 6 % ; nomlals (CFI = 3 . 0 - 4 . 7 ) , 6 8 % ; a n d champagne glasses (CFI > 4.7), 2 6 % . N e i t h e r age n o r g e n d e r of t h e d o n o r p a t i e n t c o r r e l a t e d w i t h t h e i s t h m u s w i d t h , CFI, o r M C F I (Table 3). T h e f e m o r a l c a v i t y e n l a r g e m e n t r a t e w a s less t h a n 2.0 m m p e r 1 0 - m m slice i n t e r v a l u p to t h e level 40 m m b e l o w T in a l m o s t all f e m o r a ( 9 4 % ) (Fig. 4). A b o v e t h e level T - 20, t h e e n l a r g e m e n t r a t e w a s
Results T h e m e a n s a n d s t a n d a r d d e v i a t i o n s (SDs) of t h e e n d o s t e a l d i m e n s i o n s at d i f f e r e n t levels a r e p r e s e n t e d in Table 1. T h e m e a n e n d o s t e a l n e c k a n t e v e r sion at t h e o s t e o t o m y level w a s 8 ° (range, - 1 9 ° 39°; SD, 12°). T h e a v e r a g e i s t h m u s level w a s 1 10 m m (range, 6 0 - 1 4 0 m m ; SD, 15 r a m ) b e l o w t h e lesser t r o c h a n t e r m i d p o i n t . In 7 8 % of t h e f e m o r a , t h e i s t h m u s level w a s 100 to 120 m m b e l o w t h e lesser t r o c h a n t e r m i d p o i n t . In t h e calcar f e m o r a l region, a d e n s e t r a b e c u l a r ridge p r o t r u d i n g e n d o s t e a l l y f r o m t h e p o s t e r o m e dial c o r t e x w a s d e t e c t a b l e in 9 6 % of t h e f e m o r a . T h e c o m p u t e r p r o g r a m w a s set to d e t e c t this s e p t u m , which separates the main femoral cavity from the lesser t r o c h a n t e r a n d w h i c h n a r r o w s t h e e n d o s t e a l c a v i t y at t h e b a s e of t h e n e c k (Fig. 2). T h e d e t a i l e d CFIs a n d M C F I s a r e p r e s e n t e d in Table 2 a n d t h e CFI d i s t r i b u t i o n s in Fig. 3. A c c o r d i n g to N o b l e ' s classific a t i o n [25], t h e p e r c e n t a g e of stovepipes (CFI < 3.0)
,:al,:Jr
,:~l,:~r
Fig. 2. CT scans of an osteotomy level slice (T 4-20) (above) and a lesser trochanter midpoint level slice (T) (below) of a femur. The dense trabecular calcar septum typically narrows the endosteum posteromedially.
Diversity of Proximal Femoral Medullary Canal
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T a b l e 2. Femoral Canal Flare Indices and Their Correlations (n = 50) Correlation Coefficient (r) CFI
CFI Mediolateral Neck-oriented Anteroposterior MCFI Mediolatera[ Neck-oriented Anteroposterior
MCFI Anteroposterior
Mediolateral
NeckOriented
t t
t t t
t t t
t t t
Mean
SD
Mediolateral
NeckOriented
4.2 4.3 2.3
0.88 0.93 0.51
1 0.97* 0.72*
t 1 0.73*
1
2.2 2.0 1.5
0.24 0.25 0.16
0.75* 0.67* 0.54*
0.69* 0.70* 0.53*
0.55* 0.49* 0.65*
Anteroposterior
t
0.81,
1
t t
0.66*
0.73*
1
l
SD, standard deviation; CFI, canal flare index; MCFI, m e t a p h y s e a l canal flare index. *P < .001. ¢Values repeat.
T + 20 (the s u p p o s e d b o n e i n g r o w t h r e g i o n of a n endoprosthetic femoral stem).
g r e a t e r t h a n 2 m m in 9 5 % of 1 0 - r a m slice i n t e r v a l s . T h e i s t h m u s w i d t h c o r r e l a t e d fairly w e l l w i t h t h e m e d i o l a t e r a l w i d t h at t h e level of T - 40 (r = .83, P < .001) a n d T - 20 (r = .53, P < .001) b u t p o o r l y w i t h t h e T + 20 level (r = - . 2 4 , n o t s i g n i f i c a n t ) . T h e i s t h m u s w i d t h c o r r e l a t e d fairly w e l l w i t h PCFI (r=-.72, P < .001), h o w e v e r . C o r r e l a t i o n s b e t w e e n d i m e n s i o n s at t h e levels T - 20 a n d T + 20 w e r e o n l y .43 (P = .002) m e d i o l a t e r a l l y a n d .59 (P < .001) a n t e r o p o s t e r i o r l y (Table 3). It t h u s a p p e a r e d t h a t t h e p r e d i c t i o n of o n e d i m e n s i o n f r o m a n o t h e r is u n r e l i a b l e b e t w e e n t h e levels T - 20 to
Discussion A l t h o u g h f e m o r a l e n d o s t e a l a n a t o m y is t h e basis of f e m o r a l s t e m design, o n l y a f e w d e s c r i p t i o n s of t h e a n a t o m y h a v e b e e n p u b l i s h e d . N o b l e et al. [25] demonstrated the endosteal variation, the independence between endosteal and periosteal dimensions, a n d t h e n e e d for m u l t i p l e s t e m d e s i g n s to a c h i e v e a close fit. W a l k e r a n d R o b e r t s o n [161
35
3O
25
Iii
2O II
0
'lli
°
I I 2.@2,5
2,5-3,0
3,0-3,5
3,5.4.0
i I 4,(}-4,5
4,5-5.0
5,0-5,5
.., o o 5,5-6,0
6,0"6,5
6~5-7,0
7,0-7,5
7,5-B.0
CFI
Fig, 3. Distribution of canal flare index (CFI) and neck-oriented CFI of our study is presented together with previous studies of Noble et al. [25] and of Hussmann et al. [27]. (11) our study; (O) our study, neck-oriented; (D) Noble et al (1988); (~l) Husmann et al (1997)
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The Journal of Arthroplasty Vol. 15 No. 1 January 2000
T a b l e 3. Correlation Between Endosteal Diameters and Femoral Indices and Femoral Age (n = 50)
The femoral endosteal cavity does not have a definite, s h a r p l y d e f i n e d wall: I n s t e a d , t h e r e is a gradual transition from the cavity through cancell o u s b o n e to t h e c o r t e x . In t h e d i a p h y s i s , this t r a n s i t i o n z o n e is thin, a n d t h e r e is a c l e a r e n d o s t e a l b o r d e r . In t h e m e t a p h y s i s , t h e t r a n s i t i o n z o n e is b r o a d e r . T h e i m a g e p r o c e s s i n g s y s t e m u s e d in this w o r k relies o n d i f f e r e n t t h r e s h o l d i n g m e t h o d s o n CT p i x e l v a l u e s , w h i c h reflect t h e m e c h a n i c a l p r o p erties of t h e i n d i v i d u a l b o n e s t r u c t u r e [28, 31]. N o b l e et al. [25] d e s c r i b e d t h e f e m o r a l c a v i t y o p e n i n g f r o m t h e i s t h m u s to t h e o s t e o t o m y level of m e d i o l a t e r a l CFI. In o u r study, t h e a v e r a g e m e d i o l a t eral CFI w a s l a r g e r (4.2 vs 3.9), a n d t h e p r o p o r t i o n of c h a m p a g n e g l a s s - f l u t e d f e m o r a w a s n o t a b l y g r e a t e r ( 2 4 % vs 8 % ) t h a n t h e r e s p e c t i v e figures in Noble's report. The difference was even greater w h e n o u r n e c k - o r i e n t e d CFI ( a v e r a g e 4.3, c h a m pagne glass-fluted proportion 32%) was compared w i t h N o b l e ' s CFI. T h e m a i n r e a s o n for t h e differe n c e s is p r o b a b l y t e c h n i c a l (ie, t h e l i m i t a t i o n s of s t a n d a r d x - r a y s ) , as p r e s e n t e d b y H u s m a n n et al. [27]. In a d d i t i o n , t r u e v a r i a t i o n in t h e e n d o s t e a l a n a t o m y m o s t p r o b a b l y exists b e t w e e n p o p u l a t i o n g r o u p s . In t h e s t u d y b y H u s m a n n et al. [27], t h e a v e r a g e CFI w a s s i m i l a r to o u r s (4.3), as w a s t h e p r o p o r t i o n of c h a m p a g n e g l a s s - f l u t e d f e m o r a ( 2 0 % ) . A g o o d m e t a p h y s e a l fit is o n e of t h e m a j o r goals in cementless femoral stem design [8-10,14-16,32,33]. O u r m e t h o d of a n a l y z i n g t h e s h a p e of t h e f e m o r a l m e t a p h y s e a l c a v i t y w a s s i m i l a r to t h a t of H u s m a n n et al. [27], a l t h o u g h s o m e w h a t d i f f e r e n t i n d i c e s w e r e selected, a n d o u r results w e r e in g e n e r a l
Correlation Coefficient (r)
Variables
0.83* 0.53* -0.24 -0.72* 0.43t 0.53* 0.62* 0.69* -0.08 0.01 0.10
Isthmus ML width vsT - 40 ML width Isthmus ML width vs T - 20 ML width Isthmus ML width vs T + 20 ML width Isthmus ML width vs MCFI T - 20 ML width vs T + 20 ML width T - 20 AP width vs T + 20 AP width T + 20 ML width vs T + 20 AP width T - 20 ML width vs T - 20 AP width Age vs isthmus ML width Age vs CFI Age vs MCFI
ML, mediolateral; T, trochanter; MCFI, metaphyseal canal flare index; AP, anteroposterior; CFI, canal flare index. *P< .OS. tP < .OOl.
d e s i g n e d a f e m o r a l s t e m to fit i n t o a n a v e r a g e femoral cavity based on a three-dimensional analysis. T h e y also n o t i c e d c o n s i d e r a b l e v a r i a t i o n in t h e s h a p e a n d a n g u l a t i o n of t h e f e m o r a l c a v i t y at t h e o s t e o t o m y level. R o b e r t s o n et al. [29] u s e d t h r e e d i m e n s i o n a l m o d e l i n g to d e s c r i b e a n d a n a l y z e t h e a n a t o m y of d y s p l a s t i c f e m o r a . Dai et al. [30] p o i n t e d o u t t h a t t h e effective c a v i t y a v a i l a b l e for a s t e m is s m a l l e r t h a n t h e a c t u a l c a v i t y size b e c a u s e of t h e i n c l i n a t i o n of t h e cavity" a n d t h e p r e s e n c e of t h e calcar s e p t u m d i v i d i n g t h e cavity. H u s m a n n et al. [27] d e s c r i b e d t h e c o n s i d e r a b l e v a r i a t i o n of t h e m e t a p h y s e a l e n d o s t e a l c a v i t y by using flare i n d i c e s describing the metaphyseal opening.
T+20 T+IO T T-IO T-20 T-30 T-40 T-50T-60T-70T-80T-90T-IO01"-110T-1203"-130T-140 -2
i
-
-
t~ . . . . . I
---
i
t
~mk--q
I
o
2
~ - - - ~
8
lo
12
14
~s
mm
Fig. 4. Femoral cavity enlargement rate ( m m / 1 0 - m m slice interval) presented as a box diagram: median, 25th and 75th percentiles, minimum, and maximum. Tile difference in mediolateral diameter from tile T- 150 level up to tile T- 140 level has been labeled as T- 140 ( T- 140 diameter - T- 150 diameter).
Diversity of Proximal Femoral Medullary Canal
accordance to theirs. The shape of the proximal metaphyseal cavity varies remarkably as d e m o n strated by the MCFI distribution and the poor correlations b e t w e e n dimensions at T - 20 and T + 20 levels. The goal of the second-generation cementless stems is the metaphyseal fit and fill, to achieve metaphyseal osseointegration and stress transfer. With this objective in mind, the femoral stem should be selected according to the metaphyseal shape, which is described by the MCFI rather than by the CFI, which depicts the entire proximal bone. Thus, the MCFI should have a greater influence than the CFI on stem selection. It seems therefore that a single design of cementless femoral stem would not match the variation in the shape of the proximal femur. The n u m b e r of different femoral c o m p o n e n t metaphyseal designs that would be required remains to be established. Primarily, the different designs should respond to differences in mediolateral canal opening because the metaphyseal openings (MCFI) in the mediolateral and anteroposterior planes are well correlated (r = .7) and because the posterior metaphyseal fill may be less important than the mediolateral fill in preventing rotational micromotion. Dai et al. [30] reported that a dense calcar trabeculation modified the effective cavity in the trochanteric region. In addition, Walker and Robertson [ 16] m e n t i o n e d that a p r o m i n e n t calcar was often noted in cross-sections at the osteotomy level. In accordance with these views, Brown and Ferguson [31] observed especially p r o m i n e n t stiffness elevations in the primary trabecular system of the proximal femur. The importance of the dense calcar septum in the osteotomy-level cross-sections has been neither widely studied nor discussed. During torsional loading of a hip implant (eg, during stair climbing), the femoral stem is forced into retroversion [8,34]. The calcar septum forms the posterior wall of the reamed canal in the metaphysis, and thus a wellpreserved, strong calcar septum might reduce torsional micromotion. Furthermore, intimate s t e m calcar contact may contribute to physiologic-like load transmission. It is widely acknowledged that a long rigid stem causes excessive stress shielding and calcar resorption, and the question of the optimal length of a femoral endoprosthetic stem remains controversial [35]. A stem reaching the isthmus is definitely adequate in primary hip arthroplasty. After osseointegration in the metaphysis, the distal stem might even be disadvantageous in the load-transfer mechanism and theoretically might eventually become unnecessary [35]. In conformity with this view, a short-term follow-up of a proxirnal fit stem with
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distal over-reaming and proximal hydroxyapatite coating has been reported as giving an excellent clinical o u t c o m e [36,37].
Conclusion We have performed a detailed analysis of the dimensions and shape of the endosteal cavity of the proximal femur. The shape of the metaphyseal region was variable, and there was a poor association between the dimensions in the diaphyseal cavity and in the metaphysis. A dense calcar septum was present in nearly all femora studied. The wide variation in the shapes and sizes of the proximal femoral medullary canal means that it is impossible in practice to achieve 100% cortical contact with the stem, especially in the metaphysis. The question can be raised w h e t h e r this is necessary. A good fill of the metaphysis of the femur undoubtedly increases primary stability, but cancellous bone has superior healing capacity over cortical bone. Therefore, reliable ultimate bony union might be more easily attained in a region of good quality cancellous bone. If so, maximal metaphyseal fill may not be superior to adequate fill, provided that firm primary stability and good-quality cancellous bone next to the implant are achieved. Put a n o t h e r way, maximal cortical fit in the metaphysis may be incompatible with maximal osseointegration ability on the part of the metaphyseal trabecular bone. If this is true, an optimal compromise between these two should be the design goal.
References 1. Martell JM, Pierson RH, Jacobs J J, et ah Primary total hip reconstruction with a titanium fiber-coated prosthesis inserted withotlt cement. J Bone Joint Surg Am 75:554, 1993 2. Sotereanos NG, Engh CA, Glassman AH, et al: Cementless femoral components should be from cobalt chrome. Clin Orthop 313:146, 1995 3. Havelin LI, Espehaug B, Vollset SE, Engesaeter LB: Early aseptic loosening of uncemented femoral components in primary total hip replacement. J Bone Joint Surg Br 77:11, 1995 4. Owen TD, Moran CG, Smith SR, Pinder IM: Results of uncemented porous-coated anatomic total hip replacement. J Bone Joint Surg Br 76:258, 1994 5. Neumann L, Freund KG, Sorenson KH: Long-term restihs of Charnley total hip replacement. J Bone Joint Surg Br 76:245, 1994 6. Joshi AB, Porter ML, Trail IA, et al: Long-term resuhs of Charnley low-friction arthroplasty in young patients. J Bone Joint Surg Br 75:616, 1993
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7. Malchau H, Herberts P, Ahnfeh L: Prognosis of total hip replacement in Sweden. Acta Orthop Scand 64:497, 1993 8. Burke DW, O'Connor DO, Zalenski EB, et al: Micromotion of cemented and uncemented femoral components. J Bone Joint Surg Br 73:33, 1991 9. Callaghan J J, Fulghum CS, Glisson RR, Stranne SK: The effect of femoral stem geometry on interface motion in uncemented porous-coated total hip prostheses. J Bone Joint Surg Am 74:839, 1992 10. Hua J, Walker PS: Relative motion of hip stems under load. J Bone Joint Surg Am 76:95, 1994 11. Cameron HU, Pilliar RM, MacNab I: The effect of movement on the bonding of porous metal to bone. J Biomed Mater Res 7:301, 1973 12. Pilliar RM, Lee JM, Maniatopoulos C: Observations on the effect of movement on bone ingrowth into porous-surfaced implants. Clin Orthop 208:108, 1985 13. Haddad RJ, Cook SD, Thomas KA: Biological fixation of porous-coated implants. J Bone Joint Surg Am 69:1459, 1987 14. Huiskes R, van Rietbergen B: Preclinical testing of total hip stems: the effects of coating placement. Clin Orthop 319:64, 1995 15. Weinans H, Huiskes R, Grootenboer HJ: Effects of fit and bonding characteristics of femoral stems on adaptive bone remodelling. J Biomech Eng 116:393, 1994 16. Walker PS, Robertson DD: Design and fabrication of cementless hip stems. Clin Orthop 235:25, 1988 17. Capello WN, Sallay PI, Feinberg JR: Omniflex modular femoral component. Clin Orthop 298:54, 1994 18. Whiteside LA, Arima J, White SE, et al: Fixation of the modular total hip femoral component in cementless total hip arthroplasty. Clin Orthop 298:184, 1994 19. Bargar WL, Taylor JK, Kay JF: Custom femoral components: porous hydroxyapatite. J Bone Joint Surg Br 75(suppl Ili):255, 1993 20. Reuben JD, Chang C-H, Akin JE, Lionberger DR: A knowledge-based computer-aided design and m a n u facturing system for total hip replacement. Clin Orthop 285:48, 1992 21. Stulberg SD, Stulberg BN, Wixson RL: The rationale, design characteristics, and preliminary results of a primary custom total hip prosthesis. Clin Orthop 249:79, 1989 22. Brien WW: Design aspects of custom hips. J Bone Joint Surg Br 75(suppl III):251, 1993
23. Simonet JY, Argenson JN, Aubaniac JM: The use of uncemented custom-made prostheses in high congenital dislocation of the hip. J Bone Joint Surg Br 75(suppl 1II):257, 1993 24. Lombardi AV, Mallory TH, Eberle RW, et al: Failure of intraoperatively customized non-porous femoral components inserted without cement in total hip arthroplasty. J Bone Joint Surg Am 77:1836, 1995 25. Noble PC, Alexander JW, Lindahl LJ, et al: The anatomic basis of femoral component design. Clin Orthop 235:148, 1988 26. Rubin PJ, Leyvraz PE Aubaniac JM, et al: The morphology of the proximal femur. J Bone Joint Surg Br 74:28, 1992 27. Husmann O, Rubin PJ, Leyvraz P-E el al: Threedimensional morphology of the proximal femur. J Arthroplasty 12:444, 1997 28. Laine H-J, Kontola K, Lehto MUK, et al: Image processing for femoral endosteal anatomy detecting: description and testing of a computed tomography based program. Phys Med Biol 42:673, 1997 29. Robertson DD, Essinger JR, hnura S, et al: Femoral deformity in adults with developmental hip dysplasia. Clin Orthop 327:196, 1996 30. Dai KR, An KN, Hein TJ, et al: Geometric and biomechanical analysis of the h u m a n femur. Orthop Trans 10:256, 1985 31. Brown TD, Ferguson AB: Mechanical property distributions in the cancellous bone of the h u m a n proximal femur. Acta Orthop Scand 37:429, 1980 32. Kim Y-H, Kim VEM: Resuhs of Harris-Galante cementless hip prosthesis. J Bone Joint Surg Br 74:83, 1992 33. Kim Y-H, Kim VEM: Uncemented porous-coated anatomic total hip replacement. J Bone Joint Surg Br 75:6, 1992 34. Chandler HP, Ayres DK, Tan RC, et al: Revision total hip replacement using the S-ROM femoral component. Clin Orthop 319:130, 1995 35. Huiskes R: Failed innovation in total hip replacement: Diagnosis and proposals for a cure. Acta Orthop Scand 64:699, 1993 36. Tonino AJ, Romanini L, Rossi P, et al: Hydroxyapatitecoated hip prostheses: early results from an international study. Clin Orthop 312:211, 1995 37. Donnelly W J, Kobayashi A, Freeman MA, et al: Radiological and survival comparison of four methods of fixation of a proximal femoral stem. J Bone Joint Surg Br 79:351, 1997
Diversity of Proximal Femoral Medullary Canal H.-J. L a i n e , M D , M. U. K. L e h t o , M D , P h D , a n d T. M o i l a n e n , M D , P h D
A b s t r a c t : To analyze the endosteal dimensions of the proximal femoral medullary canal, we studied 50 cadaver femora using computed tomography and a border detection method from 20 mm above tile lesser trochanter, the osteotomy level, down to the isthmus. We documented the presence o1 a dense calcar septum in 96% of femora studied. In addition to mediolateral, anteroposterior, and neck-oriented dimensions, we calculated canal flare indices (CFIs) between the osteotomy level and the isthmus and metaphyseal canal flare indices (MCFIs) between tile osteotomy level and the level 20 m m below the lesser trochanter midpoint to describe the shape of the endosteal cavity. With respect to the canal opening, the anteroposterior and mediolateral planes parallelled each other over the entire region as indicated by the high correlation ( r = . 7 , P < . 0 0 1 ) between the CFIs in these directions. The prediction of one dimension from another was unreliable in the metaphyseal region, where bone ingrowth is supposed to occur in a femoral prosthesis. The MCFI seemed to be the best parameter to distinguish the various types of proximal femoral canal shapes. We have described the variability of the proximal femoral endosteal dimensions in detail and find that the wide variation in the shape and size of the proximal femoral medullary canal means that it is almost impossible to achieve 100% cortical contact with the stem, especially in the metaphysis. K e y w o r d s : hip, arthroplagty, femoral anatomy, medullary canal, endoprosthesis design.
Good clinical results and midterm survival rates of cementless total hip arthroplasties have been published [1,2], although failures of endoprosthetic design still occur, even in the short-term [3,4]. Cemented arthroplasty offers excellent long-term survival rates [5], but problems are still encountered with young patients [6,7]. Thus, improvements in cementless hip arthroplasty aim at durable results even in younger, physically active patients. Femoral stern micromotion studies have emphasized the importance of metaphyseal endosteal stem fit in the reduction of torsional motion [8-10], enabling bony ingrowth and thus stable biologic
fixation [ 11-13]. Finite-element studies [ 14,15] and photoelastic coating strain analysis [16] have also demonstrated the importance of metaphyseal fit in achieving physiologic-like implant-bone load transfer and in minimizing stress shielding and disadvantageous bone remodeling. Modular [17,18] and custom-made [19-21] femoral prostheses have been designed to optimizefit andfill, to minimize micromotion, and thus to maximize bone ingrowth. Early radiologic and clinical results have been promising, but problems have been encountered (eg, intraoperative femoral fractures [20,22,23]), and disappointing results of Cementless, nonporous custommade femoral components have been reported [24]. Noble et al. [25] demonstrated that there is no universal shape for the femoral cavity. They described great variability of endosteal anatomy and poor predictability of endosteal dimensions by using standard radiographs of 200 cadaver femora. Rubin et al. [26] used computed tomography (CT) combined with a border detection computer program to
From the Division of Orthopaedics, Department of Surgery, Tampere University Hospital and Medical School, University of Tampere. Tampere, Finland. Submitted October 29, 1997; accepted November 30, 1998. Reprint requests: H.-J. Laine, MD, Tampere University Hospital, Department of Surgery, PO Box 2000, SF-33521 Tampere, Finland. C6pyrighl © 2000 by Churchill Livingstone® 0883- 5403/00/1501-0014510.OO/O
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study femoral m o r p h o l o g y and reported similar results. H u s m a n n et al. [27] described the endosteal m o r p h o l o g y of the proximal f e m u r focusing on the variation of the m e t a p h y s e a l and calcar region. It seems clear that a single cementless stem design, even if several sizes are available, does not fit all femoral cavities [25,27]. The purpose of the present study was to analyze the dimensions m e a s u r e d from the endosteal surface of the cadaver proximal femoral m e d u l l a r y canal in the diaphysis and in the m e t a p h y s i s to d e t e r m i n e w h e t h e r it is rational to aim at perfect fit and fill in cementless femoral c o m p o n e n t fixation or if the variability of the inner surface is so great that 100% cortical contact cannot in practice be achieved.
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tal 2 - m m - t h i c k slices in 5 - m m intervals u p w a r d from 20 m m below the zero-level and 14 4 - m m thick slices in 10-ram intervals d o w n w a r d from 20 m m b e l o w the zero-level w e r e taken. The endosteal surface was detected from the CT data by a borderdetection computer program, which was developed especially for this purpose as previously described [28]. The femoral cavity was divided into 3 structurally different parts. The diaphyseal bone of the femoral shaft has a clear border b e t w e e n the cortical and cancellous bone. The lesser t r o c h a n t e r area has posteromedially a calcar s e p t u m of nearly cortical density dividing the femoral cavity from the lesser trochanter. Above the lesser trochanter, the metaphyseal trabecular b o n e transforms gradually to cortical bone, especially on the lateral side. The detectable lateral cortex normally extends 20 to 40 m m above the lesser t r o c h a n t e r midpoint [28]. The longest mediolateral, anteroposterior, and oblique endosteal diameters of the slices w e r e computed. The level of the slice (a value given as the distance below the zero-level) that had the n a r r o w est mediolateral d i a m e t e r (ie, the isthmus width) was defined as the isthmus level. At the o s t e o t o m y level (20 m m above T), the endosteal neck anteversion was defined as the angle b e t w e e n the longest oblique (ie, the neck oriented) d i a m e t e r and the horizontal mediolateral d i a m e t e r (Fig. 1). The pres-
Materials and Methods CT was p e r f o r m e d in 50 cadaver femora (21 left and 29 right, from separate white individuals, 36 male and 14 female, average age at death 70 years [range, 38-87 years] ). Institutional Ethical C o m m i t tee approval was obtained for the use of cadaver femora. Two femora w e r e placed on thermoplastic (Plexiglas) at the same time so that their vertical axes were parallel to the table axis and their rotations were neutral. They w e r e placed with their lesser t r o c h a n t e r midpoints at the same vertical level (referred to as the zero-level, T). Sixteen horizon-
Ac T level E
T-20
H G ~
level
I
isthmus lever
J
A B C 0 E F G H i
= = = = = = = = =
the the the the the the the the the
longest longest longest longest longest longest longest longest longest
med~oloteral diometer, T+20 level onterioposterlor d;orneter, T+20 level obl;que diometer, T+20 level medlo$oterol diometer. T - 2 0 level onterloposterlor diometer. T - 2 0 level oblique diometer. "1"-20 level medloloterol diometer, isthmus level onter~oposterlot diometer, isthmus level oblique diometer, ; s t h m u s level
G = medloloteral conol (lore index. CFI H = onterioposterior conol (Io~e index. CFIop G = collum oriented Conol flore index. CFIco
A ~/ ~ = medioloterol proximol conol flore index, PCFI B onterloposter;or proximol conol flore index. PCFtop C // O = collum oriented prox;mol t o n a l (lore index, PCFI¢o
Fig. 1. The longest mediolateral, anteroposterion and oblique diameters were computed for each slice. Canal flare indices (CFIs) were calculated for each femur by the diameters of the isthmus level slice and the osteotomy level slice (20 mm above lesser trochanter midpoint, T 4. 20). Metaphyseal canal flare indices (MCFIs) were calculated for each femur by the diameters of the slice of 20 mm below ( T - 20) the lesser trochanter midpoint and the osteotomy level slice (T 4- 20). Endosteal neck anteversion angle (e0 was measured from the osteotomy level slice of each femur.
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e n c e of a d e n s e c o r t i c a l - l i k e s e p t u m in t h e c a l c a r f e m o r a l a r e a w a s also s t u d i e d . Canal flare indices (CFIs) w e r e c a l c u l a t e d as t h e ratio b e t w e e n t h e m e d i o l a t e r a l c a n a l w i d t h at t h e o s t e o t o m y level a n d t h e i s t h m u s w i d t h [25]. Anteroposterior CFIs w e r e c a l c u l a t e d s i m i l a r l y f r o m t h e a n t e r o p o s t e r i o r d i m e n s i o n s . Neck-oriented CFIs w e r e d e t e r m i n e d as t h e ratio b e t w e e n t h e l o n g e s t o b l i q u e d i m e n s i o n at t h e o s t e o t o m y level a n d t h e i s t h m u s w i d t h (Fig. 1). T h e s e figures w e r e d e f i n e d to d e scribe t h e f e m o r a l c a v i t y o p e n i n g f r o m t h e i s t h m u s to t h e o s t e o t o m y level. To specify t h e v a r i a b i l i t y of t h e f e m o r a l m e t a p h y seal o p e n i n g , t h e metaphyseal canalflare index (MCFI) w a s d e t e r m i n e d (ie, t h e ratio b e t w e e n m e d i o l a t e r a l c a n a l w i d t h 20 m m a b o v e a n d 20 m m b e l o w t h e lesser t r o c h a n t e r m i d p o i n t IT] ). Anteroposterior MCFIs a n d t h e neck-oriented MCFIs w e r e d e t e r m i n e d in a s i m i l a r f a s h i o n (Fig. 1). To a n a l y z e t h e r e g u l a r i t y of t h e s h a p e of t h e m e t a p h y s e a l f e m o r a l cavity, w e c a l c u l a t e d t h e b i v a r i a t e c o r r e l a t i o n s of t h e d i m e n sions in p r o x i m a l slices (from 20 m m b e l o w to 20 m m a b o v e T). To s t u d y t h e r a t e of o p e n i n g f r o m d i a p h y s i s to m e t a p h y s i s , w e d e t e r m i n e d t h e e n l a r g e m e n t r a t e of t h e f e m o r a l c a v i t y (ie, t h e i n c r e a s e of t h e m e d i o l a t eral d i a m e t e r p e r 1 0 - m m slice i n t e r v a l ) . D e s c r i p t i v e statistics, d i s t r i b u t i o n graphics, a n d P e a r s o n l i n e a r c o r r e l a t i o n s w i t h s c a t t e r p l o t s w e r e u s e d to d e t e r m i n e t h e typical f e m o r a l e n d o s t e a l d i m e n s i o n s a n d their variation.
T a b l e 1. Proximal Femoral Endosteal D i m e n s i o n s (ram) at Different Levels (n = 50) Mediolateral Dimensions
Anteroposterior Dimensions
Longes! Oblique Dimensions
Level
Mean
SD
Mean
SD
Mean
SD
T + 20 T + 15 T + 10 T+ 5 T T - 10 T - 20 T - 30 T - 40 T - 60 Isthmus T - 150
45.42 40.43 35.49 31.51 28.73 25.03 20.41 17.74 15.73 13.43 11.06 12.65
4.46 4.59 4.23 4.05 3.21 2.58 2.14 1.93 1.97 2.12 1.88 2.03
31.39 29.79 28.55 26.95 25.58 23.85 20.71 18.27 16.77 15.92 14.09 14.82
3.45 3.18 3.06 2.95 2.86 2.32 2.47 2.39 2.56 2.67 2.81 2.64
47.09 41.95 37.53 33.95 31.01 27.58 23.19 20.08 18.68 17.33 14.79 15.59
4.98 4.76 4.31 4.22 3.65 2.53 2.74 2.52 2.51 2.65 2.96 2.69
SD, standard deviation; T, trochanter.
w a s 6 % ; nomlals (CFI = 3 . 0 - 4 . 7 ) , 6 8 % ; a n d champagne glasses (CFI > 4.7), 2 6 % . N e i t h e r age n o r g e n d e r of t h e d o n o r p a t i e n t c o r r e l a t e d w i t h t h e i s t h m u s w i d t h , CFI, o r M C F I (Table 3). T h e f e m o r a l c a v i t y e n l a r g e m e n t r a t e w a s less t h a n 2.0 m m p e r 1 0 - m m slice i n t e r v a l u p to t h e level 40 m m b e l o w T in a l m o s t all f e m o r a ( 9 4 % ) (Fig. 4). A b o v e t h e level T - 20, t h e e n l a r g e m e n t r a t e w a s
Results T h e m e a n s a n d s t a n d a r d d e v i a t i o n s (SDs) of t h e e n d o s t e a l d i m e n s i o n s at d i f f e r e n t levels a r e p r e s e n t e d in Table 1. T h e m e a n e n d o s t e a l n e c k a n t e v e r sion at t h e o s t e o t o m y level w a s 8 ° (range, - 1 9 ° 39°; SD, 12°). T h e a v e r a g e i s t h m u s level w a s 1 10 m m (range, 6 0 - 1 4 0 m m ; SD, 15 r a m ) b e l o w t h e lesser t r o c h a n t e r m i d p o i n t . In 7 8 % of t h e f e m o r a , t h e i s t h m u s level w a s 100 to 120 m m b e l o w t h e lesser t r o c h a n t e r m i d p o i n t . In t h e calcar f e m o r a l region, a d e n s e t r a b e c u l a r ridge p r o t r u d i n g e n d o s t e a l l y f r o m t h e p o s t e r o m e dial c o r t e x w a s d e t e c t a b l e in 9 6 % of t h e f e m o r a . T h e c o m p u t e r p r o g r a m w a s set to d e t e c t this s e p t u m , which separates the main femoral cavity from the lesser t r o c h a n t e r a n d w h i c h n a r r o w s t h e e n d o s t e a l c a v i t y at t h e b a s e of t h e n e c k (Fig. 2). T h e d e t a i l e d CFIs a n d M C F I s a r e p r e s e n t e d in Table 2 a n d t h e CFI d i s t r i b u t i o n s in Fig. 3. A c c o r d i n g to N o b l e ' s classific a t i o n [25], t h e p e r c e n t a g e of stovepipes (CFI < 3.0)
,:al,:Jr
,:~l,:~r
Fig. 2. CT scans of an osteotomy level slice (T 4-20) (above) and a lesser trochanter midpoint level slice (T) (below) of a femur. The dense trabecular calcar septum typically narrows the endosteum posteromedially.
Diversity of Proximal Femoral Medullary Canal
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T a b l e 2. Femoral Canal Flare Indices and Their Correlations (n = 50) Correlation Coefficient (r) CFI
CFI Mediolateral Neck-oriented Anteroposterior MCFI Mediolatera[ Neck-oriented Anteroposterior
MCFI Anteroposterior
Mediolateral
NeckOriented
t t
t t t
t t t
t t t
Mean
SD
Mediolateral
NeckOriented
4.2 4.3 2.3
0.88 0.93 0.51
1 0.97* 0.72*
t 1 0.73*
1
2.2 2.0 1.5
0.24 0.25 0.16
0.75* 0.67* 0.54*
0.69* 0.70* 0.53*
0.55* 0.49* 0.65*
Anteroposterior
t
0.81,
1
t t
0.66*
0.73*
1
l
SD, standard deviation; CFI, canal flare index; MCFI, m e t a p h y s e a l canal flare index. *P < .001. ¢Values repeat.
T + 20 (the s u p p o s e d b o n e i n g r o w t h r e g i o n of a n endoprosthetic femoral stem).
g r e a t e r t h a n 2 m m in 9 5 % of 1 0 - r a m slice i n t e r v a l s . T h e i s t h m u s w i d t h c o r r e l a t e d fairly w e l l w i t h t h e m e d i o l a t e r a l w i d t h at t h e level of T - 40 (r = .83, P < .001) a n d T - 20 (r = .53, P < .001) b u t p o o r l y w i t h t h e T + 20 level (r = - . 2 4 , n o t s i g n i f i c a n t ) . T h e i s t h m u s w i d t h c o r r e l a t e d fairly w e l l w i t h PCFI (r=-.72, P < .001), h o w e v e r . C o r r e l a t i o n s b e t w e e n d i m e n s i o n s at t h e levels T - 20 a n d T + 20 w e r e o n l y .43 (P = .002) m e d i o l a t e r a l l y a n d .59 (P < .001) a n t e r o p o s t e r i o r l y (Table 3). It t h u s a p p e a r e d t h a t t h e p r e d i c t i o n of o n e d i m e n s i o n f r o m a n o t h e r is u n r e l i a b l e b e t w e e n t h e levels T - 20 to
Discussion A l t h o u g h f e m o r a l e n d o s t e a l a n a t o m y is t h e basis of f e m o r a l s t e m design, o n l y a f e w d e s c r i p t i o n s of t h e a n a t o m y h a v e b e e n p u b l i s h e d . N o b l e et al. [25] demonstrated the endosteal variation, the independence between endosteal and periosteal dimensions, a n d t h e n e e d for m u l t i p l e s t e m d e s i g n s to a c h i e v e a close fit. W a l k e r a n d R o b e r t s o n [161
35
3O
25
Iii
2O II
0
'lli
°
I I 2.@2,5
2,5-3,0
3,0-3,5
3,5.4.0
i I 4,(}-4,5
4,5-5.0
5,0-5,5
.., o o 5,5-6,0
6,0"6,5
6~5-7,0
7,0-7,5
7,5-B.0
CFI
Fig, 3. Distribution of canal flare index (CFI) and neck-oriented CFI of our study is presented together with previous studies of Noble et al. [25] and of Hussmann et al. [27]. (11) our study; (O) our study, neck-oriented; (D) Noble et al (1988); (~l) Husmann et al (1997)
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T a b l e 3. Correlation Between Endosteal Diameters and Femoral Indices and Femoral Age (n = 50)
The femoral endosteal cavity does not have a definite, s h a r p l y d e f i n e d wall: I n s t e a d , t h e r e is a gradual transition from the cavity through cancell o u s b o n e to t h e c o r t e x . In t h e d i a p h y s i s , this t r a n s i t i o n z o n e is thin, a n d t h e r e is a c l e a r e n d o s t e a l b o r d e r . In t h e m e t a p h y s i s , t h e t r a n s i t i o n z o n e is b r o a d e r . T h e i m a g e p r o c e s s i n g s y s t e m u s e d in this w o r k relies o n d i f f e r e n t t h r e s h o l d i n g m e t h o d s o n CT p i x e l v a l u e s , w h i c h reflect t h e m e c h a n i c a l p r o p erties of t h e i n d i v i d u a l b o n e s t r u c t u r e [28, 31]. N o b l e et al. [25] d e s c r i b e d t h e f e m o r a l c a v i t y o p e n i n g f r o m t h e i s t h m u s to t h e o s t e o t o m y level of m e d i o l a t e r a l CFI. In o u r study, t h e a v e r a g e m e d i o l a t eral CFI w a s l a r g e r (4.2 vs 3.9), a n d t h e p r o p o r t i o n of c h a m p a g n e g l a s s - f l u t e d f e m o r a w a s n o t a b l y g r e a t e r ( 2 4 % vs 8 % ) t h a n t h e r e s p e c t i v e figures in Noble's report. The difference was even greater w h e n o u r n e c k - o r i e n t e d CFI ( a v e r a g e 4.3, c h a m pagne glass-fluted proportion 32%) was compared w i t h N o b l e ' s CFI. T h e m a i n r e a s o n for t h e differe n c e s is p r o b a b l y t e c h n i c a l (ie, t h e l i m i t a t i o n s of s t a n d a r d x - r a y s ) , as p r e s e n t e d b y H u s m a n n et al. [27]. In a d d i t i o n , t r u e v a r i a t i o n in t h e e n d o s t e a l a n a t o m y m o s t p r o b a b l y exists b e t w e e n p o p u l a t i o n g r o u p s . In t h e s t u d y b y H u s m a n n et al. [27], t h e a v e r a g e CFI w a s s i m i l a r to o u r s (4.3), as w a s t h e p r o p o r t i o n of c h a m p a g n e g l a s s - f l u t e d f e m o r a ( 2 0 % ) . A g o o d m e t a p h y s e a l fit is o n e of t h e m a j o r goals in cementless femoral stem design [8-10,14-16,32,33]. O u r m e t h o d of a n a l y z i n g t h e s h a p e of t h e f e m o r a l m e t a p h y s e a l c a v i t y w a s s i m i l a r to t h a t of H u s m a n n et al. [27], a l t h o u g h s o m e w h a t d i f f e r e n t i n d i c e s w e r e selected, a n d o u r results w e r e in g e n e r a l
Correlation Coefficient (r)
Variables
0.83* 0.53* -0.24 -0.72* 0.43t 0.53* 0.62* 0.69* -0.08 0.01 0.10
Isthmus ML width vsT - 40 ML width Isthmus ML width vs T - 20 ML width Isthmus ML width vs T + 20 ML width Isthmus ML width vs MCFI T - 20 ML width vs T + 20 ML width T - 20 AP width vs T + 20 AP width T + 20 ML width vs T + 20 AP width T - 20 ML width vs T - 20 AP width Age vs isthmus ML width Age vs CFI Age vs MCFI
ML, mediolateral; T, trochanter; MCFI, metaphyseal canal flare index; AP, anteroposterior; CFI, canal flare index. *P< .OS. tP < .OOl.
d e s i g n e d a f e m o r a l s t e m to fit i n t o a n a v e r a g e femoral cavity based on a three-dimensional analysis. T h e y also n o t i c e d c o n s i d e r a b l e v a r i a t i o n in t h e s h a p e a n d a n g u l a t i o n of t h e f e m o r a l c a v i t y at t h e o s t e o t o m y level. R o b e r t s o n et al. [29] u s e d t h r e e d i m e n s i o n a l m o d e l i n g to d e s c r i b e a n d a n a l y z e t h e a n a t o m y of d y s p l a s t i c f e m o r a . Dai et al. [30] p o i n t e d o u t t h a t t h e effective c a v i t y a v a i l a b l e for a s t e m is s m a l l e r t h a n t h e a c t u a l c a v i t y size b e c a u s e of t h e i n c l i n a t i o n of t h e cavity" a n d t h e p r e s e n c e of t h e calcar s e p t u m d i v i d i n g t h e cavity. H u s m a n n et al. [27] d e s c r i b e d t h e c o n s i d e r a b l e v a r i a t i o n of t h e m e t a p h y s e a l e n d o s t e a l c a v i t y by using flare i n d i c e s describing the metaphyseal opening.
T+20 T+IO T T-IO T-20 T-30 T-40 T-50T-60T-70T-80T-90T-IO01"-110T-1203"-130T-140 -2
i
-
-
t~ . . . . . I
---
i
t
~mk--q
I
o
2
~ - - - ~
8
lo
12
14
~s
mm
Fig. 4. Femoral cavity enlargement rate ( m m / 1 0 - m m slice interval) presented as a box diagram: median, 25th and 75th percentiles, minimum, and maximum. Tile difference in mediolateral diameter from tile T- 150 level up to tile T- 140 level has been labeled as T- 140 ( T- 140 diameter - T- 150 diameter).
Diversity of Proximal Femoral Medullary Canal
accordance to theirs. The shape of the proximal metaphyseal cavity varies remarkably as d e m o n strated by the MCFI distribution and the poor correlations b e t w e e n dimensions at T - 20 and T + 20 levels. The goal of the second-generation cementless stems is the metaphyseal fit and fill, to achieve metaphyseal osseointegration and stress transfer. With this objective in mind, the femoral stem should be selected according to the metaphyseal shape, which is described by the MCFI rather than by the CFI, which depicts the entire proximal bone. Thus, the MCFI should have a greater influence than the CFI on stem selection. It seems therefore that a single design of cementless femoral stem would not match the variation in the shape of the proximal femur. The n u m b e r of different femoral c o m p o n e n t metaphyseal designs that would be required remains to be established. Primarily, the different designs should respond to differences in mediolateral canal opening because the metaphyseal openings (MCFI) in the mediolateral and anteroposterior planes are well correlated (r = .7) and because the posterior metaphyseal fill may be less important than the mediolateral fill in preventing rotational micromotion. Dai et al. [30] reported that a dense calcar trabeculation modified the effective cavity in the trochanteric region. In addition, Walker and Robertson [ 16] m e n t i o n e d that a p r o m i n e n t calcar was often noted in cross-sections at the osteotomy level. In accordance with these views, Brown and Ferguson [31] observed especially p r o m i n e n t stiffness elevations in the primary trabecular system of the proximal femur. The importance of the dense calcar septum in the osteotomy-level cross-sections has been neither widely studied nor discussed. During torsional loading of a hip implant (eg, during stair climbing), the femoral stem is forced into retroversion [8,34]. The calcar septum forms the posterior wall of the reamed canal in the metaphysis, and thus a wellpreserved, strong calcar septum might reduce torsional micromotion. Furthermore, intimate s t e m calcar contact may contribute to physiologic-like load transmission. It is widely acknowledged that a long rigid stem causes excessive stress shielding and calcar resorption, and the question of the optimal length of a femoral endoprosthetic stem remains controversial [35]. A stem reaching the isthmus is definitely adequate in primary hip arthroplasty. After osseointegration in the metaphysis, the distal stem might even be disadvantageous in the load-transfer mechanism and theoretically might eventually become unnecessary [35]. In conformity with this view, a short-term follow-up of a proxirnal fit stem with
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distal over-reaming and proximal hydroxyapatite coating has been reported as giving an excellent clinical o u t c o m e [36,37].
Conclusion We have performed a detailed analysis of the dimensions and shape of the endosteal cavity of the proximal femur. The shape of the metaphyseal region was variable, and there was a poor association between the dimensions in the diaphyseal cavity and in the metaphysis. A dense calcar septum was present in nearly all femora studied. The wide variation in the shapes and sizes of the proximal femoral medullary canal means that it is impossible in practice to achieve 100% cortical contact with the stem, especially in the metaphysis. The question can be raised w h e t h e r this is necessary. A good fill of the metaphysis of the femur undoubtedly increases primary stability, but cancellous bone has superior healing capacity over cortical bone. Therefore, reliable ultimate bony union might be more easily attained in a region of good quality cancellous bone. If so, maximal metaphyseal fill may not be superior to adequate fill, provided that firm primary stability and good-quality cancellous bone next to the implant are achieved. Put a n o t h e r way, maximal cortical fit in the metaphysis may be incompatible with maximal osseointegration ability on the part of the metaphyseal trabecular bone. If this is true, an optimal compromise between these two should be the design goal.
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