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Bone Remodeling is Different in Metaphyseal and Diaphyseal-fit Uncemented Hip Stems

Saito, Jun, MD, PhD; Aslam, Nadim, MD,FRCSC*; Tokunaga, Kenji, MD*; Schemitsch, Emil H, MD,FRCSC*; Waddell, James P, MD,FRCSC*

Clinical Orthopaedics and Related Research®: October 2006 - Volume 451 - Issue - p 128-133
doi: 10.1097/01.blo.0000224045.63754.a3
SECTION II: ORIGINAL ARTICLES: Hip
Free

Femoral component stability in uncemented total hip arthroplasties depends on periprosthetic bone remodeling. Stem design is an important factor influencing bone remodeling, however the design that promotes the most bone remodeling is unclear. We examined metaphyseal and diaphyseal-fit stems to determine the effect of stem design on bone remodeling and stability. Twenty-three patients who had total hip arthroplasties (28 hips) with metaphyseal-fit stems were matched with 27 patients (32 hips) who had uncemented total hip arthroplasties with diaphyseal-fit stems. We assessed preoperative radiographs for canal fill, canal shape, and bone quality. We then assessed postoperative radiographs for periprosthetic bone remodeling including spot welds, cortical hypertrophy, and pedestal formation. Patients were examined clinically using a modified Harris hip score. Patients with metaphyseal stems had increased cortical hypertrophy 1 year postoperatively. However, there was no functional difference 2 years postoperatively. Both stem designs resulted in bone remodeling by 2 years postoperatively with similar clinical results.

Level of Evidence: Level III Therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

From the *Division of Orthopaedic Surgery, Department of Surgery, St. Michael's Hospital, University of Toronto, Toronto, Ontario, Canada; and the Department of Orthopaedic Surgery, University of Occupational and Environmental Health, Kitakyushu, Japan.

Each author certifies that his institution has approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research, and that informed consent was obtained.

One of the authors (JS) has received funding from the Nakayama Foundation for Human Science (Bunkyo-ku, Tokyo, Japan).

Revised: October 8, 2005; April 13, 2006

Accepted: April 20, 2006

Received: March 8, 2005

Correspondence to: Jun Saito, MD, PhD, Department of Orthopaedic Surgery, University of Occupational and Environmental Health, 1-1 Iseigaoka, Yahatanishi-ku, Kitakyushu 807-8555, Japan. Phone: 81-93-691-7444; Fax: 81-93-692-0184; E-mail: j-saito@med.uoeh-u.ac.jp.

Loosening of cemented femoral components was a major concern before modern cementing techniques.22,29 Uncemented femoral components originally were developed to reduce loosening and eventually gained wide acceptance.

Despite that acceptance, failure rates did not exceed those reported in some series of cemented implants, and therefore, more uncemented femoral components were developed to improve stability, osseous integration, and durability.8,21,24

In uncemented total hip arthroplasties (THA), long-term femoral component stability depends on bone remodeling including bone ingrowth in implants with porous surfaces.5 Bone remodeling is affected by many factors related to host bone and component design. Stem design is one of the most important factors influencing clinical results.30,32 There are numerous types of stems designs. Diaphyseal-fit stems aim for solid biologic fixation in the narrow portion of the diaphysis and sustain loads mainly in the distal portion of the stem.4,6 Metaphyseal-fit stems allow stress transfer in a proximal-to-distal fashion, mainly from the tapered proximal portion.23,24 Metaphyseal-fit stems were developed to avoid the complications associated with diaphyseal-fit stems such as stress shielding, thigh pain, and osteolysis.23,24

The radiographic and clinical outcomes of uncemented stems have been studied.10,32 In uncemented cylindrical diaphyseal-fit stems, the presence of spot welds around the porous surface was considered a major sign of osseous integration. Spot welds and proximal bone resorption were found in most hips that had stable fixation.7 However, tapered metaphyseal-fit stems were commonly associated with distal cortical hypertrophy without spot welds or severe resorptive bone changes.25 Because periprosthetic bone remodeling between diaphyseal and metaphyseal-fit stem designs rarely was compared, 20 the influence of the femoral component design on periprosthetic bone remodeling and stability in uncemented THAs is unknown.

We asked whether the periprosthetic bone remodeling and clinical results differed in metaphyseal and diaphyseal-fit uncemented stems. We compared: (1) periprosthetic bone remodeling induced by metaphyseal and diaphysealfit stems over 2 years; (2) other radiographic changes such as stem subsidence, malalignment, and radiolucent formation; (3) clinical results of both stems; (4) radiographic fit-and-fill characteristics of metaphyseal and diaphysealfit stems; and (5) proximal canal shape and preoperative bone quality.

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MATERIALS AND METHODS

We retrospectively evaluated 23 patients (28 hips) who had primary uncemented THAs with metaphyseal-fit stems and 27 patients (32 hips) who had primary uncemented THAs with diaphyseal-fit stems at our institution from 1998 to 2002 (Table 1). The patients were matched for age, gender, diagnosis, and duration of followup. Only patients meeting the matched criteria were included. All patients were followed up regularly at our institution after primary uncemented THAs. Their preoperative diagnosis was primary osteoarthritis (OA). Patients with inflammatory arthritis, avascular necrosis, posttraumatic arthritis, or arthritis secondary to childhood diseases were excluded to obtain a uniform population (Table 1). The average age of the patients at the time of surgery was 57.1 years (range, 50-67 years) in the metaphyseal-fit group and 58.8 years (range, 41-71 years) in the diaphyseal-fit group. We established a minimum followup of 2 years. The average radiographic and clinical followups were 2.7 years (range, 2-6 years) in the metaphyseal-fit group and 3.2 years (range, 2-5 years) in the diaphyseal-fit group.

TABLE 1

TABLE 1

We assessed patients using radiographic and clinical examinations preoperatively, at 6 weeks, 3 months, 6 months postoperatively, and yearly thereafter. Anteroposterior (AP) radio-graphs of the pelvis and cross-table lateral radiographs of the hips were taken at each followup. Two research fellows (NA and KT) not involved in the patients' surgeries were blinded to the results and independently reviewed the serial radiographs and measured the radiographic parameters including periprosthetic bone remodeling. Each observer measured all radiographs three times. The intraobserver or interobserver variations were averaged as representative of the outcome.

The surgeries were done using a posterior approach to the hip and standard preparation methods. Patients received a metaphyseal-fit stem (Synergy®, Smith and Nephew, Memphis, TN) or a diaphyseal-fit stem (Echelon®, Smith and Nephew). The Synergy® stem is titanium alloy (Ti6Al4V) and the Echelon® stem is cobalt-chromium. Both stems have a proximal porous coating that is identical in bead size with no hydroxyapatite (HA) coating. The Synergy® stem has a tapered design and aims for three-point fixation. The Echelon® stem has a cylindrical shape for diaphyseal biologic fixation. Distal fixation in the Echelon® stem is achieved by flutes, which secure excellent rotational stability but do not provide a surface for bone ingrowth. The surgical technique for the Synergy® stem needs tapered reamers that match the taper of the stem. After reaming, we performed broaching to enlarge the metaphyseal portion of the femur to insert the prosthesis. The stem was inserted so it was rigid in the metaphyseal bone. For the Echelon® stem, we used cylindrical reamers slightly smaller than the diameter of the stem to ensure excellent distal fixation. Broaching was used only to expand the metaphyseal portion of the femur to allow prosthesis insertion.

We used preoperative radiographs to evaluate proximal canal shape and canal-to-calcar (CC) ratio. Bone quality was determined with the cortical index for cortical bone3,25 and the Singh index for trabecular bone.28 The postoperative radiographic evaluation included metaphyseal fit and isthmus fill. The former was the percentage of the metaphysis occupied by the stem at the level of the lesser trochanter, and the latter was the percentage of the diaphysis occupied by the stem 3 cm proximal to the tip of the stem. Periprosthetic bone remodeling was expressed as spot welds, cortical hypertrophy, and pedestal formation.7,25 Spot-weld formation was defined as new bone bridging the endosteum and the porous surface of the implant around the greater trochanter.7 Cortical hypertrophy was defined as new bone of cortical density resulting in increased cortical thickness at the diaphysis.7,25 A halo pedestal was defined as a thin radiodense line surrounding the tip of the stem.25 An incomplete shelf of dense bone at the tip of the stem was defined as a shelf pedestal.25

We also evaluated progressive subsidence, malalignment, radiolucent line formation, and calcar rounding (Figs 1 and 2). Progressive subsidence for more than 2 years was considered clinically important.7,25 Alignment was considered neutral if varus or valgus angulation was within 3° of the femoral axis.16 A radiolucent line wider than 2 mm around the porous coating was considered positive.17,18,25 These criteria were based on previous reports suggesting these radiographic signs were indicators of instability or loosening.7,16,17,25 Calcar rounding was assessed as atrophic change of the calcar.7,25 We evaluated subsidence, angulation, radiolucent lines, and calcar rounding 2 years postoperatively. Bone remodeling and other radiographic changes were assessed using Gruen zones.9 Patients were evaluated clinically using the modified Harris hip score (HHS) at preoperative and postoperative followups. We calculated pain and functional scores as a part of the HHS, and thigh pain was recorded at each interval. We also recorded complications such as intraoperative calcar fracture, hip dislocation, or early revision.

Fig 1A

Fig 1A

Fig 2A

Fig 2A

All data were reported as means and standard error (SE) of the mean. We performed analysis of variance (ANOVA) followed by a Fisher-type adjustment or a Mann-Whitney U test to evaluate differences of continuous data between two groups. We used chi square or Fisher's exact tests for statistical evaluation of categorical variables. A p value less than 0.05 was considered significant.

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RESULTS

Patients with metaphyseal stems had a greater frequency of cortical hypertrophy postoperatively (Fig 3) although metaphyseal and diaphyseal-fit stems showed bone remodeling with no difference in spot welds or pedestal formation 2 years postoperatively (Fig 3). There were increases (p < 0.05) in the frequency and timing of spot welds with the metaphyseal-fit stem 3 months and 6 months postoperatively, but there was no difference in the frequency of spot welds 1 year and 2 years postoperatively (Fig 3A). Spot welds were located mainly in Gruen Zone 1. Cortical hypertrophy was greater (p < 0.05) at 6 months, 1 year, and 2 years postoperatively with the metaphyseal-fit stems compared with the diaphyseal-fit stems (Fig 3B). Cortical hypertrophy only occurred in Gruen Zones 3 and 5. Halo pedestal formation was greater (p < 0.05) at 6 months in patients with metaphyseal-fit stems, but not at 1 year and 2 years postoperatively (Fig 3C). Neither group had a shelf pedestal.

Fig 3A

Fig 3A

We observed no progressive subsidence with either stem type. Only three patients (10.7%) in the metaphysealfit stem group and three patients (9.4%) in the diaphysealfit stem group showed transient subsidence, but the subsidence was 2 mm or less. Twenty-seven metaphyseal-fit stems were implanted in the neutral position and one stem (3.6%) was placed in varus. Twenty-four diaphyseal-fit stems were implanted in the neutral position and eight stems (25%) were placed in varus. Varus angulation of the stem did not increase after 2 years and was 3° or less. There were no radiolucent lines around the stem in either group. Calcar rounding occurred in 25 hips (90.0%) with metaphyseal-fit stems and in 20 hips (62.5%) with diaphyseal-fit stems.

The average preoperative HHS were similar in metaphyseal-fit stems (48.9 ± 2.7 points) and diaphyseal-fit stems (54 ± 2.8 points). The average HSS for metaphyseal-fit stems (90.6 ± 1.5) and diaphyseal-fit stems (91.7 ± 1.7) also were similar at the latest followup. The prevalence of transient thigh pain was similar in the two groups (3.6% and 6% with metaphyseal and diaphyseal-fit stems, respectively) (Table 2). No patient had aseptic loosening leading to femoral component revision.

TABLE 2

TABLE 2

The proximal canal fill was greater (p < 0.01) in metaphyseal-fit stems (77%) compared with diaphyseal-fit stems (61%) (Table 1). However, the distal canal fill was greater (p < 0.01) in diaphyseal-fit stems (94%) compared with metaphyseal-fit stems (83%) (Table 1).

There was no difference in the CC ratio, cortical index, and Singh index between the metaphyseal-fit and diaphy-seal-fit stem groups. The proximal canal shape and bone quality were similar in both groups (Table 1).

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DISCUSSION

Long-term stability of an uncemented femoral component depends on reactive bone remodeling.5 Stem design is one of the most important factors influencing bone remodeling.30,32 Metaphyseal and diaphyseal-fit stem designs are common femoral component designs, but their effects on bone remodeling are unclear. We evaluated how periprosthetic bone remodeling and stability differed in metaphyseal-fit and diaphyseal-fit stems in uncemented THAs. We assessed radiographic and clinical results of metaphysealfit and diaphyseal-fit stem designs. Bone remodeling occurred differently in each group at 2 years postoperatively, but clinical results were similar.

Our study has several limitations. Component material is an important factor for bone remodeling in addition to implant design. Cobalt-chromium stems are stiffer than titanium stems, which ostensibly reduces proximal bone resorption and cortical bone thinning around the distal portion of the stem.1,12 Titanium has been shown to achieve superior bone ingrowth into porous surface as compared with cobalt-chrome.12,13,17 Results of a previous study showed that uncemented titanium stems had more shelf pedestal formation than uncemented cobalt-chromium stems, although clinical results were equal.17 The different materials may have affected bone remodeling, but we could not evaluate the effects with our data. Our data also are limited because we radiographically evaluated bone remodeling and canal fill without using dual energy xray absorptiometry scanning or computed tomography.8,19,26 Although the radiographs were standardized, differences in the technique could have resulted in underestimation or overestimation of bone remodeling. There also are limitations assessing bone remodeling by radiographs. Radiographic evaluations depend on reviewers' observations and interpretations. All surgical procedures and treatments were standardized, but operations were performed by two surgeons (ES and JW). Although useful for understanding adaptive bone remodeling,24 our followups were too short to determine long-term outcomes.

Spot welds around the proximal porous surface are considered a major sign of osseous integration.7 The presence of spot welds has a high specificity for bony fixation, although the absence of spot welds has a low specificity for unstable fixation.7 One study showed that spot welds and resorptive bone remodeling were not reliable radio-graphic indicators of stability in a tapered titanium stem.25 We found an increased frequency of spot welds in metaphyseal-fit stems at 3 months and 6 months postoperatively, but no difference in the frequency at 1 year and 2 years postoperatively. Unstable fixation did not occur in either group, and clinical results were similar 1 year postoperatively. Early appearance of spot welds might indicate greater bone ingrowth with good initial stability.7,31

The clinical importance of cortical hypertrophy is not well understood, although its frequency is reported as 14% to 46%.11,14,15,25,27 Cortical hypertrophy increased at 6 months, 1 year, and 2 years postoperatively in metaphyseal-fit stems compared with diaphyseal-fit stems. The frequency of cortical hypertrophy gradually increased to 45% in metaphyseal-fit stems versus 9.3% in diaphyseal-fit stems at 2 years postoperatively. Our results were consistent with those of a previous study of bone mineral density in patients with cobalt-chrome straight AML stems (DePuy, Warsaw, IN) and patients with titanium-wedge CLS (Zimmer, Warsaw, IN) stems.8 Postoperative bone loss in the CLS group was consistently less in all Gruen zones compared with the AML group, and the CLS stem had a net gain in Gruen Zone 5 because of femoral hyper-trophy.8 Cortical hypertrophy might occur because of a lateral bending moment created by inserting the stem into a curved femur.2,25 The tapered shape and decreased rigidity of the metaphyseal-fit stem might allow easy transmission of a lateral bending moment at the distal aspect of the stem. Some authors have mentioned that cortical hypertrophy was associated with an unstable stem,18,32 but we found no evidence supporting that claim. Porous coating is one of the important factors for bone remodeling. However, cortical hypertrophy occurred with both stem designs in the distal part of the femoral component. Bone remodeling occurred in the distal part of metaphyseal and diaphyseal-fit stems, despite no porous coating in this region.

A halo pedestal is considered a sign of micromotion.17,25 Halo pedestals were common in both groups, especially in the group with metaphyseal-fit stems. It was unclear whether this result indicated metaphyseal-fit stems moved more than the diaphyseal-fit stems, because we did not find any other radiographic or clinical evidence concerning micromotion. Partial thin bone formations around the tip of the stem were classified as halo pedestals if they existed or if they grew continuously for a few years. However, that categorization might have led to overestimation.

Some authors have suggested that progressive stem subsidence and progressive radiolucent lines predict instability and loosening.7,24,25 Progressive subsidence for more than 2 years did not occur in either group. Similarly, radiolucent lines did not occur. These results suggest both stems obtained good stability.

Femoral component malalignment has been associated with poor outcomes, especially varus placement.18,33 One stem (3.6%) was placed in varus angulation in the metaphyseal-fit group and eight stems (25%) were placed in varus angulation in the diaphyseal-fit group. However, there were no signs of loosening or clinical complications, which may be because varus angulation was within 3°. The varus angulation did not progress during the 2-year followup.

Calcar rounding is reported as a sign of bone atrophy of the proximal femur because of stress shielding.7,25 There was increased calcar rounding in metaphyseal-fit stems initially, but there was no difference between the groups at 2 years postoperatively. This is in contrast to previous radiographic reviews in which the authors reported that large, cylindrical femoral components cause more proximal bone atrophy.4,7 Calcar atrophy can be assessed more accurately by measuring bone mineral density in addition to radiographic evaluation.23

After 1 year postoperatively, the only difference between the two groups was cortical hypertrophy, which was greater in patients with metaphyseal-fit stems. Both stems showed bone remodeling with no differences in spot welds or pedestal formation. By 2 years postoperatively, there were no functional differences between the groups. A longer-term followup is necessary to further determine the relationship between the clinical and radiographic results.

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Acknowledgments

We thank T. Nakamura, MD, PhD, H. Ohnishi, MD, PhD, and R. Zdero, PhD, for critical reading of the manuscript and helpful advice. We also thank J. Pan, MD, and J. Morton, RN, for technical assistance.

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References

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