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.
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.
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.
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).
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.
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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