Stem length and canal filling in uncemented custom-made total hip arthroplasty

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1 International Orthopaedics (SICOT) (1999) 23: Springer-Verlag 1999 ORIGINAL PAPER Takashi Sakai Nobuhiko Sugano Takashi Nishii Keiji Haraguchi Takahiro Ochi Kenji Ohzono Stem length and canal filling in uncemented custom-made total hip arthroplasty Accepted: 2 June 1999 Abstract We reviewed 60 custom-made femoral components of two different lengths : 125 mm (group A) and 100 mm (group B), in order to investigate the relationship between stem length and canal filling in uncemented custom-made total hip arthroplasty. There were no statistical differences between the two groups in age, gender, height, body weight, canal flare index, or bowing angle of the femur. Postoperatively there was no statistical difference between the two groups in the proximal canal filling, but significant difference in the distal canal filling (75.5% vs 85.8% on the anteroposterior view and 76.0% vs 82.5% in the lateral view, P<0.001). The distal canal filling inversely correlated with the ratio of the proximal portion and the distal portion of the stem curvature on the lateral view (lateral curve ratio of the stem, P=0.002). We conclude that superior filling at both the proximal and the distal levels can be obtained by using 100-mm custom made components with a small lateral curve ratio. Résumé. Pour étudier la relation existant entre la longueur de la tige et le remplissage du canal dans une arthroplastie totale de la hanche effectuée sur mesure, nous avons réexaminé 60 éléments fémoraux de deux longueurs différentes: 125 mm (groupe A) et 100 mm (groupe B). Que ce soit l áge, le genre, la hauteur, le poids du corps, l indice d érasement du canal ou l angle de courbure du fémur, il n existait pas de différences statistiques entre les deux groupes. Alors qu il n y avait pas de différences statistiques post-opératoires entre les deux groupes dans le remplissage du canal proximal, il y avait par contre une différence significative dans le remplissage du canal distal (75.5% par rapport á 85.8% sur une vue antéropostérieure et 76.0% par rapport á 82.5% sur une vue latérale, P<0.001). Le remplissage du canal distal se T. Sakai ( ) N. Sugano T. Nishii K. Haraguchi T. Ochi K. Ohzono Department of Orthopedic Surgery, Osaka University Medical School, 2-2 Yamadaoka, Suita, , Osaka, Japan tsakai@ort.med.osaka-u.ac.jp Tel.: , Fax: rattachait inversement avec le rapport de la portion proximale et de la portion distale de la courbure de la tige sur la vue latérale (rapport de la courbure latérale, P=0.002). On en conclut que le remplissage supérieur aux niveaux proximaux et distaux peut étre obtenu en utilisant des éléments sur mesure de 100 mm avec un faible rapport de la courbure latérale. Introduction Optimal fit (surface area of the implant in direct contact with the endosteal surface) and optimal canal filling (the percentage of the cross-sectional area of the femoral canal occupied by the prosthesis) are important factors for stable fixation in uncemented total hip arthroplasty (THA) [1 3], as well as is surface finish [1, 4, 5]. To achieve optimal fit and fill in patients with variable femoral geometry, especially in osteoarthritis secondary to hip dysplasia, custom-made femoral prostheses [6 12] or modular prostheses [13, 14] are preferable to off-theshelf components. Since 1994 we have been using uncemented custommade femoral components, mainly for patients with secondary osteoarthritis of the hip. A curved stem was designed to obtain the maximum fill in the medullary canal. Although stem length is one of the variable factors in custom-made components, there have been few reports of stem length in anatomic prostheses [15, 16]. We reviewed patients who underwent THA using custommade femoral components with two different stem lengths in order to investigate the relationship between stem length and canal filling. Patients and methods Between January 1994 and July 1997, 99 custom-made femoral components were introduced into 77 patients with acetabular dysplasia or congenital dislocation of the hip and secondary degenerative disease (76 patients : 97 hips) or osteonecrosis (one patient : both hips) (first generation). The initial 28 patients with 30 hips

2 220 Table 1 Preoperative profile First generation Second generation Mann-Whitney U-test group (A) group (B) Stem length (mm) P< Lateral curve ratio of the stem a 2.53±0.78 ( ) 1.61±0.59 ( ) Total number of hips Gender (male:female) 4:24 5:20 Age at operation (years) 53±7 (43 73) 55±7 (43 70) P=0.678 Height (cm) 151±7 ( ) 154±6 ( ) P=0.161 Weight (kg) 55±7 (45 68) 52±7 (38 74) P=0.110 Canal flare index 3.72±0.51 ( ) 3.65±0.67 ( ) P=0.478 Bowing angle (degree) 15.1 ±6.2 ( ) 16.5 ±42 ( ) P=0.204 a The ratio of the proximal portion and the distal portion of the stem curvature on the lateral view. Each value was represented by mean or mean±standard deviation (4 males, 24 females) with secondary osteoarthritis were selected as Group A. Custom-made femoral components (Cremascoli, Milano, Italy) were made with the aid of computerized tomography (CT). The inner and outer contours of the femur were digitized on serial 5 mm pitch axial CT images. The maximum implantable femoral component with a 125 mm stem was designed and made from titanium-alloy (Ti-6Al-4V) with a sand-blasted surface. The femoral canal was prepared using a custom broach that precisely replicated the shape of the prosthesis. In November 1997 the stem length was reduced to 100 mm, to obtain a better fit and canal filling. By August 1998, 37 custom made femoral components had been introduced into 31 patients with secondary osteoarthritis of the hip (second generation). 25 of these patients with 30 hips (5 males and 20 females) were selected as group B, and matched Group A in age, gender, height and body weight. The comparative figures are shown in Table 1. To estimate femoral geometry, the canal flare index on the anteroposterior (AP) view was calculated and the bowing angle on the lateral view (17) was measured on preoperative xrays (Fig. 1). The canal filling seen on the AP and lateral xrays 3 weeks after surgery was measured at the following levels : the lesser trochanter, 1 cm below the lesser trochanter, and 1 cm above the stem tip (Fig 2). This data was digitally scanned on a MacIntosh computer and the canal filling was measured using the NIH image ver.1.61/ppc. The ratio of the proximal and distal portions of the stem curvature on the lateral view was defined as the lateral curve ratio (LCR) and was compared with the canal filling (Fig. 3). The Mann-Whitney U-test and Spearman rank correlation coefficient were used for statistical analysis, and a P-value of less than 0.05 was considered significant. Results Canal flare index and bowing angle are shown in Table 1. There was no statistical difference between the two groups in this preoperative profile. Canal filling is shown in Table 2. At the level of the lesser trochanter, the mean canal filling on the AP view was 87.4% for Group A and 89.3% for Group B, and on the lateral view it was 91.9% for Group A and 92.2% for Group B. At 1 cm below the lesser trochanter the corresponding figures on the AP view were 91.0% for Group A and 93.0% for Group B and on the lateral view 91.1% for Group A and 92.3% for Group B. There was thus no statistical difference in the proximal filling between the two groups. At 1 cm above the tip of the stem, the mean filling on the AP view was 75.5% for Group A and 85.8% for Group B (P<0.0001), and on the lateral view it was 76.0% for Group A and 82.5% for Group B (P=0.0004). This represented a significant difference in the distal filling between the two groups. There was no correlation between the canal flare index and canal filling on the AP view, or between the bowing angle and the canal filling on the lateral view (Table 3). The mean LCR was 2.53 for Group A and 1.61 for Group B (P<0.0001) (Table 1). There was an inverse correlation between LCR and canal filling at 1 cm below the lesser trochanter on the AP view (P=0.030, Table 4), between LCR and distal filling on the AP view (P=0.002, Fig. 4A), and between LCR and distal filling on the lateral view (P=0.002, Fig. 4B). Discussion Uncemented custom-made femoral components have been advocated for enhanced fit and fill for variable hip geometry [6, 10, 11]. However some clinical studies have failed to show that custom-made prostheses significantly improve implant longevity, suggesting that the surface finish was not optimal for uncemented implantation [7, 8]. The surface of the custom-made components used in the present study was sand-blasted, the efficacy of which has been previously reported [4]. Both optimal canal filling and satisfactory surface finish can be expected to lead to greater longevity of the prosthesis. Using custom-made components with various shapes, we noticed in some patients that good canal filling could not be achieved with a long, custom-made femoral component. We therefore investigated the relationship between stem length and canal filling. There was no statistical difference between the two groups in canal filling at the proximal level (the lesser trochanter and 1 cm below). At the distal level, canal filling with the short stem was significantly superior to that with the long stem. For the initial design of the custom-made femoral components, an attempt was made to achieve maximal canal filling both at the proximal and the distal levels. However, on the simulation of insertion

3 221 Fig. 4A,B LCR related to distal canal filling on the AP view (A) and on the lateral view (B) Fig. 1A,B Preoperative radiographic assessment. Canal flare index (CFI) (A) and bowing angle (α) (B) Fig. 2A,B Canal filling. Anteroposterior view (A) and lateral view (B) Fig. 3 Lateral curve ratio of the stem (LCR) of the femoral component, it was found that the longer the custom-made component the more difficult it was to insert, due to the curvature of the femur. As Horne indicated [18], the three-dimensional curvature of the femoral canal precludes the use of a stem that completely fills the canal because such a stem could not be inserted. To achieve more than 80% distal canal filling on both AP and lateral views, the ideal LCR on the lateral view was less than 1.75 (Fig. 4A,B). Thus, the LCR may be an important factor for curved anatomical stems to achieve good canal filling at the distal level. A short stem with a small LCR may achieve good canal filling at both proximal and distal levels. Although the ideal is to determine the stem length in proportion to the body profile of each patient, and espe-

4 222 Table 2 Canal filling First generation Second generation Mann-Whitney U-test group (A) group (B) (n=30 hips) (n=30 hips) On the anteroposterior view (%) 87.4±6.4 ( ) 89.3±4.2 ( ) P= cm below the lesser trochanter level (%) 91.0±4.0 ( ) 93.0±4.1 ( ) P= cm above the stem tip level (%) 75.5±7.2 ( ) 85.8±6.4 ( ) P< On the lateral view (%) 91.9±2.8 ( ) 92.2±4.0 ( ) P= cm below the lesser trochanter level (%) 91.1±4.0 ( ) 92.3±4.1 ( ) P= cm above the stem tip level (%) 76.0±7.0 ( ) 82.5±6.0 ( ) P= Each value was represented by mean±standard deviation Table 3 Femoral profile and canal filling Table 4 Lateral curve ratio of the stem (LCR) and canal filling Spearman rank correlation coefficient (n=60 hips) Canal flare index vs canal filling on the AP view P= cm below the lesser trochanter level P= cm above the stem tip level P=0.913 Bowing angle vs canal filling on the lateral view P= cm below the lesser trochanter level P= cm above the stem tip level P=0.223 Spearman rank correlation coefficient (n=60 hips) LCR vs canal filling on the AP view P= cm below the lesser trochanter level P=0.030* 1 cm above the stem tip level P=0.002* LCR vs canal filling on the lateral view P= cm below the lesser trochanter level P= cm above the stem tip level P=0.002* *P<0.05 cially to the length of the femur, we reduced the stem length from 125 mm to 100 mm by removing the parts whose distal filling showed less than 80% on the AP and the lateral X-rays. We tried to verify the improved distal filling with a short stem, observed on computer simulation, in the clinical part of the present study. It was found that short stems more often achieved good canal filling at both proximal and distal levels, thus decreasing micromotion, thigh pain, and loosening [19, 20] and ensuring uniform stress transfer [21]. As to the advantages of a short stem, Noble et al showed that such a stem (90 mm) yielded very low values for rotational and translational micromotion, which were significantly smaller than those observed with longer stemmed components (130 and 170 mm) in cadavera [15]. Huiskes showed that longer stems had higher rigidity and produced more stress in the diaphysis, which might be a cause of thigh pain [16]. However, even with a short stem, good canal filling at the distal level may lead to proximal stress shielding [22], so that, clinical results and implant longevity should be reviewed carefully. Sugano [23] reported lower rates of thigh pain and migration when a fully poroussurfaced short stem was used with a minimum stem length of 90 mm. While we have no data for stems less than 90 mm long, we believe that 100 mm stems are satisfactory. In conclusion, while both long-stemmed (125 mm) and short-stemmed (100 mm) custom-made prostheses could achieve good canal filling at the proximal level, better canal filling at the distal level was achieved with short-stemmed prostheses. Better filling at the proximal and the distal levels can thus be obtained by using 100 mm custom-made components with a small LCR. References 1. Boune RB, Rorabeck CH, Burkart BC, Kirk PG (1994) Ingrowth surfaces. Plasma spray coating to titanium alloy hip replacement. Clin Orthop 298: Clarke HJ, Jinnah RH, Cox QG, Curtis MJ (1992) Computerized templating in uncemented total hip arthroplasty to assess component fit and fill. J Arthroplasty 7: Engh CA, Glassman AH, Suthers KE (1990) The case for porouscoated hip implants. The femoral side. Clin Orthop 261: Feighan JE, Goldberg VM, Davy D, Parr JA, Stevenson S (1995) The influence of surface-blasting on the incorporation of titanium-alloy implants in a rabbit intramedullary model. J Bone Joint Surg [Am] 77: Geesink RGT, Hoefnagels NHM (1995) Six-year results of hydroxyapatite-coated total hip replacement. J Bone Joint Surg [Br] 77: Barger WL (1989) Shape the implant to the patient. A rationale for the use of custom-fit cementless total hip implants. Clin Orthop 249: 73 78

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