10 years results of an uncemented metaphyseal fit modular stem in elderly patients.

BACKGROUND: There are concerns with regard to the femoral fixation in cementless total hip arthroplasty in elderly patients. We report a retrospective analysis of clinical and radiological results of uncemented metaphyseal fit modular stem in elderly patients irrespective of anatomic characterstics of proximal femur.
MATERIALS AND METHODS: This study reviews the outcomes of 60 primary hip replacements using a metaphyseal fit modular stem (third-generation Omniflex stem) conducted in 54 patients, of age 75 years or older. After a mean follow-up of 10,4 years, complete clinical and radiographic records were available for 52 hips of 48 patients. The patients were evaluated by Harris Hip Score (HHS).
RESULTS: There was a significantly improved pain score and Harris Hip Score (41,6 to 83,2). Six stems (11.53%) were revised: four because of periprosthetic fracture; one stem was well fixed, but presented a large osteolytic lesion in the metaphyseal area and the last stem was revised because of aseptic loosening. Stem survival taking aseptic loosening as the end-point was 98%. Bone atrophy in the proximal femur caused by stress shielding was observed in 39 stems (75%), but there was no case of subtrochanteric stress shielding. Moreover, atrophy appeared within two years postoperatively, with no extension thereafter.
CONCLUSIONS: We achieved good clinical and radiographic results by uncemented metaphyseal fit femoral stem regardless of patient's age and femoral canal type.

INTRODUCTION

There is much controversy over many issues related to the use of a femoral stem when conducting a total hip arthroplasty in an elderly patient, as there are no convincing data to support the choice of a cemented or non-cemented stem as a function of patient age.12 Several authors have reported good results using both cemented34 and cementless5–7 stems in these patients. For the use of the latter, initial fixation is the key to achieving optimal primary stability.89 Fixation is determined by the type of implant used, the anatomy and quality of the femoral canal, and the surgical technique.10 Cementless stem designs range from highly porous cylindrical stems of initial diaphyseal fixation, to stems initially fixed to the metaphyseal bone that may or may not be anatomical. Both designs achieve long-lasting fixation.1112 However, diaphyseal fixation prostheses induce marked proximal femoral atrophy and a thigh pain that has shadowed their clinical results.13

In 1997, our Department began to implant the modular metaphyseal-fixed cementless stem OMNIFLEX. The aim of this retrospective study is to assess the clinical and radiographic results obtained in the long-term, in elderly patients (older than 75 years), irrespective of the anatomic characteristics of the proximal femur.

MATERIALS AND METHODS

This retrospective case-series was approved by our Institution's Scientific Research Board. Patients were enrolled if they were 75 years or older and had radiographically-proven primary or secondary coxarthrosis, having undergone total hip arthroplasty with the implant of a cementless modular femoral stem. All patients had an anesthetic evaluation (ASA classification), and those with ASA 4 or higher were rejected from undergoing the surgical procedure. Patients were excluded if they had received a femoral stem after an intracapsular fracture.

The sample size was estimated on the basis of a precision of 4%, a long-lasting fixation of primary hip replacements using a metaphyseal fit modular of 95%, after 10 years of follow-up, an 80% confidence level, and losses of 10%. As a result, 54 patients were included in the study.

Between January 1998 and December 1998, a total of 114 total hip arthroplasties were performed in 105 patients at our institute. The mean age of patients was 78 years (75 – 86), and were implanted with 60 femoral components (six bilateral). Mean follow-up was 10,4 years (range, 10 – 11 years). Four patients (five hips) died before ten years from surgery due to unrelated cause. Two patients (one undergoing bilateral arthroplasty) were lost to follow-up. This left 48 patients whose data were retrospectively assessed and in whom 52 femoral components had been implanted. Of these 48 patients, 21 were men (43.75%) and 27 women (56.25%). 80% (n = 42) had been diagnosed with primary arthrosis, 8% (n = 4) with rheumatoid arthritis, and the the rest with (n = 6) idiopathic avascular necrosis.

All the patients received the same cementless modular stem. This stem formed part of the OMNIFLEX hip system (Howmedica Osteonics, Allendale, NJ), and was one of the early cementless designs. The modular femoral component system was designed to address the issue of proximodistal size mismatch and favor proximal stress transfer. Initially, the femoral component was made of Ti alloy with a double-wedge configuration and sintered proximal Ti bead pads, which were later replaced with a proximal circumferential arc-deposition with a 50 μm layer of hydroxyapatite (HA) coating (third-generation). The distal two-thirds of the component was tapered, to increase flexibility, with a grit-blasted coating, and the cylindrical polished CoCr tip was modular, to fit several distal canal diameters. A modular collar was available for placement after stem insertion, but in none of the present cases was this collar used. The intraoperative requirement for the use of the OMNIFLEX stem was that the test rasp could be stably fitted otherwise a cemented stem was used.

The acetabular component used was a semispherical Titanium Omnifit PSL (Howmedica Osteonics, Allendale, NJ) with a 10° polyethylene rim, to increase the superior-lateral cover and a 28 mm chrome-cobalt head.

In all cases, the surgical approach was the modified Hardinge transgluteal anterolateral, with the patient in a lateral position.14 A second-generation cephalosporin was administered half-an-hour preoperatively and during the first two postoperative days. On the second postoperative day, touch-toe weight-bearing walking with two crutches was allowed. Partial weight-bearing was encouraged at four weeks, gradually increasing to full weight-bearing at approximately 12 weeks postoperatively.

The patients were evaluated at three, six, and 12 months postoperatively and yearly thereafter, according to the Harris Hip Score.15 At their latest follow-up visit, all the patients underwent a detailed physical evaluation (BJT) and were checked for pain in the middle third of the thigh.

Radiographic follow-up was performed at the same time as the clinical follow-up and two of us (BJT, SZ) analyzed them at each follow-up. Preoperative assessment was based on anteroposterior and lateral views of the hip, including half the femur. Femoral morphology was determined by calculating the femoral canal index, defined as the ratio of the intracortical width of the femur, at a point 20 mm proximal to the tip of lesser trochanter and at the canal isthmus.10 Ratios lower than 3.0 indicated canals in the shape of a tube (type C femur), ratios of 3.0 – 4.7 indicated a normal canal (type B femur), and those greater than 4.7, a canal shaped like a champagne glass (type A femur).10

Using the radiographs obtained three months postoperatively, the proximal portion of the stem fill was measured at the central level of the lesser trochanter. This calculation of the metaphyseal filling was undertaken according to the modified criterion established by Dorr16 [Figure 1]. The position of the stem was recorded in relation to the anatomic axis of the femur (varus, valgus or neutral).

Figure 1

A line diagram showing various parameters

The fixation state of the femoral component was determined on an anteroposterior (AP) and lateral view of the hip obtained on the most recent follow-up visit. For this, we used the criteria established by Engh17 for femoral components with proximal porous coatings and classified the stems as stable with bone ingrowth (osseointegration), unstable with fibrous ingrowth. Analysis of the zones of bone ingrowth was undertaken according to the zones of Gruen,18 with Gruen zones 1, 2, 6, 7, 8, and 14 corresponding to the porous implant surface.

Bone atrophy in the proximal femur, due to stress shielding, was examined in the AP view of the hip obtained in the most recent follow-up, and graded according to the severity classification of Engh.19

Finally, we evaluated polyethylene (PE) wear using the technique described by Livermore.20 The mean PE wear value was determined in the anteroposterior hip radiograph by calculating the difference between PE thickness in the most recent follow-up and the immediate postoperative, and dividing this figure by the number of years of follow-up. PE wear in the revised stems was correlated with the initial metaphyseal fill of the implant, to try to relate the reduced initial metaphyseal filling with the passage of debris particles due to wear. Similarly, we assessed whether the femurs showed more atrophy (osteopenia) on the anteroposterior view due to the shielding, which displayed greater PE wear.

Statistical analysis

A statistical analysis was performed using the SPSS software (version 12.0). A descriptive study of the sample was made. Qualitative variables were described in terms of frequency distribution, and compared using the c2 or the Fisher's exact test. Quantitative variables were described by reference to their mean and standard deviations, and compared using the Student's t or Mann-Whitney test. For more than two quantitative variables, the analysis of variance (ANOVA) or the Kruskal-Wallis test was used.A Kaplan-Meier survivorship analysis was used to assess the life span of the OMNIFLEX stem, defining the end point as stem revision for loosening.21

RESULTS

Clinical outcome

Of the 52 total hip prostheses implanted in 48 patients, six (11,5%) required stem revision after a mean follow-up time of 10,4 years [Table 1]. Of all stems one (1,9%) was revised for aseptic loosening one-and-a-half years after implantation, and another, seven-and-a-half years postoperatively, for osteolytic lesion involving the proximal part of the femur. The rest of the stem revisions were done for periprosthetic fractures.

Table 1

Profiles of the six patients undergoing stem revision

During follow-up, thigh pain relating to two femoral components (3.8%) was noted in two patients. This pain did not limit the activity of the patients or require revision of the stem. Both patients had type A femurs. The Kaplan-Meier stem survival rate (95% confidence interval) with stem revision for loosening as the end point was 98.3 ± 0,45% at 10,4 years. Harris scores increased from a preoperative mean of 41.6 (29 – 52) to 83.2 (67 – 92) in the last follow up (P < 0.05).

Radiographic assessment

According to the preoperative radiograph, four femurs were classified as type A, 44 type B, and four type C, using the Noble Scheme.10 One stem underwent subsidence during the second postoperative year. This was a type B femur in a male patient with osteoarthritis [Figure 2]. The patient was revised with a revision stem (Restoration HA, Stryker). The mean percentage of the metaphyseal fill was 75.46 (SD= 7.3) calculated on the AP view and 77.6 (SD = 7.4) on the lateral view, with differences lacking significance (P > 0.05) for the different femoral canals [Table 2].

Figure 2

(a) The postoperative radiograph of right hip with thigh (anteroposterior view) at three months in a 77-year-old man with secondary osteoarthritis received an OMNIFLEX stem showing a metaphyseal fill less than 70%. (b) Postoperative radiograph at 18 months showing subsidence of the stem. This patient underwent stem revision surgery

Table 2

Percentage metaphyseal fill

The stem revised because of subsidence showed a metaphyseal fill of 69% in the AP and of 71% in the axial view, differing significantly from the percentages recorded for the non-revised stems.

The second stem was revised for loosening of the acetabular component. During surgery, a non-contained osteolytic lesion involving the metaphyseal area was observed that compromised the future stability of the femoral component. The stem was therefore retrieved and replaced with a revision stem (Restoration HA, Stryker) [Figure 3].

Figure 3

Radiograph showing an Omniflex stem implanted in an 83-year-old man seven years ago. Note the presence of osteolysis in the metaphyseal area and evidence of acetabular component loosening. Both components were revised

Alignments of the femoral component were neutral in 40 hips (76.9%), valgus in seven hips (13.46%), and varus in five hips (9.61%). The stem revised because of subsidence showed a varus alignment of 5°. No significant correlation was detected between the stem position and stem loosening [Figure 4].

Figure 4

Last follow-up (10 years and 4 months) anteroposterior radiograph taken in a 77-year-old woman. Note, the femoral stem shows a valgus alignment yet the hip shows radiographic evidence of stable fixation

Fixation of the femoral component was classified as stable with a bone ingrowth in 48 hips (92.3%) (four in type A, 41 in type B, and three in type C femurs), stable fibrous in three hips (5.7%) (two in type B, one in type C femurs), and unstable in one hip (1.9%) (type B). Given that the mean femoral fill in all types of femoral canals was > 70% and the stem that was revised because of subsidence showed a fill < 70%, we could say the stems that were stable by bone ingrowth differed significantly from the unstable stem (P = 0.01), but not with respect to the stems stable by fibrous ingrowth (P > 0.05).

Radiographic cancellous condensation indicating osseointegration was observed mainly in Gruen stem zones 2 and 6, and in smaller measure in zones 1 and 7 of the anteroposterior view, and in zones 8 and 14 of the lateral view.

Bone atrophy due to stress shielding appeared in the proximal femur in 39 stems (75%). These femurs had a stem that was well integrated in the host tissue or showed stable fibrous ingrowth. Of these 39 femurs, 21 (53.84%) showed atrophy of severity grade I; 12 (30.76%) grade II; and six (15.38%) grade III. No cases of grade IV atrophy were detected. All atrophies due to stress shielding appeared in the first two years of follow-up and did not progress thereafter. The three stems stable because of fibrous ingrowth showed grade I atrophy. Femurs exhibiting atrophy grade III corresponded to the stems stable by bone ingrowth; three type A and three type B.

Mean PE wear at 10,4 years was 1.1 mm (range 0.3 – 3.1). Mean wear was not greater in the stem revised because of subsidence, but was worse in the stem revised for acetabular loosening. The range of PE wear in femurs showing bone atrophy due to stress shielding was no greater than that recorded in those without signs of atrophy (P > 0.05).

DISCUSSION

The proportion of persons older than 75 years ascribed to our hospital is slowly but steadily growing.22 There are few reports of the use of cementless femoral prostheses in this age group5–7132324 [Table 3].

Table 3

Results of cementless THAs in elderly patients

Concerns about the use of cementless stems in elderly persons are essentially based on two facts. First, the high incidence of intraoperative fractures, and second, failure to achieve good bone ingrowth in the stem. The latter is the outcome of a poorer bone tissue quality, as this factor diminishes with age.2526 Accordingly, many surgeons consider the use of cementless stems in elderly persons inappropriate, on the grounds than an initial stem stability cannot be achieved.2728

No intraoperative fractures were recorded in our series. The essential prerequisite for the use of a cementless stem was that the test rasp could be stably fitted without the need for a cortical contact around the entire stem periphery. Four periprosthetic fractures, three type B1 and one type B2 according to the Vancouver classification, were recorded during the follow-up, and the stems were revised because the fracture could affect the stability of the implant. In some cases we used long modular stem revision (Restoration modular system, Stryker), and in other cases we used long stem revision (Restoration HA). Stem revision for oblique or transverse B1 fractures is now considered as a viable treatment modality, as this fracture configuration is difficult to control with single plating29 [Figure 5].

Figure 5

(a) Radiograph of an 83-year-old man showing an oblique Vancouver B1-type fracture around the stem. The stem was unstable, and a revision with a long modular stem was done (b)

No detrimental effects of fixation were observed in terms of pain and functionality. The incidence of pain, albeit low, recorded in the middle third of the thigh (3.8%) could be due to the diameter of the distal centralizer placed at the tip of the stem. The size selected for this centralizer was a diameter of at least 1 mm less than the last rasp used to assess the canal size.

Noble et al.10 established a radiographic classification scheme for femur canals, which was further developed by Dorr et al.30 according to radiographic and histomorphometric criteria. Thus, following Noble's radiographic scheme and his premise that a single stem design would not be compatible with the different femoral canal types, we assessed the behavior of a single stem type, regardless of the morphology of the femoral canal.

In March 1997, the OMNIFLEX hip system was introduced in our Orthopedic Surgery Unit as the only uncemented femoral stem. Implants coated proximally with HA have provided good clinical outcomes, mostly in young patients.123132 However, only a few studies have assessed the performance of stems whose metaphyseal portion is coated with HA, in elderly patients. The present study has analyzed patients older than 75 years undergoing surgery in 1998 (52 stems implanted in 48 patients) after the initial learning curve. The use of stems had started several months prior to this date.

Our series of patients revealed a predominance of type B femoral canals. One stem was revised because of mechanical loosening. The initial metaphyseal fill was less than 70%, and its alignment was varus by 5°. Dorr16 and Martell33 observed that metaphyseal fitted stems needed a fill exceeding 90% that required cortical contact. Our series of patients showed a mean metaphyseal fill of less than 90%, as we tried to avoid cortical contact over the entire periphery, to reduce the risk of intraoperative periprosthetic fracture. This could explain why HA coated stems need a lesser proximal fill.3435

Four of the femurs were type C, for which the use of cemented stem would normally be recommended.3637 Hence, obtaining sufficient stability for an optimal outcome of the implant of a cementless prosthesis, in this type of femur, is a particular challenge. Although the number of cases assessed here is too low to draw any valid conclusions, it would seem that the osteoconductive properties of HA might help stem ingrowth in type C femurs [Figure 6].

Figure 6

A postoperative radiograph at 10, 2 years reveals a well-fixed, primary, cementless stem in the proximal femur of type C morphology

The results obtained with the modular OMNIFLEX system have been reviewed by several authors in younger patients, but to our knowledge, no series describing the outcome of this stem have been published in elderly patients. Capello, Kitamura, and Ito383940 reported poor results using first generation stems, which featured a proximally non-circumferential porous tip. However, Takahashi35 obtained good results with the use of second generation stems, with arc deposition of pure titanium on the surface of the proximal circumference, and with third generation models whose circumferential porous tip was coated with HA. No revision stem was required, and 97% of all stems showed bone ingrowth fixation. We report one revised stem (1.9%) due to aseptic loosening. Thus there are clear differences among the results obtained with the first, second, and third generation stems, with the latter showing the best outcomes. In effect, a HA-coated metaphyseal portion promotes bone ingrowth.

In our series, 22% of the stems showed a valgus or varus alignment, with no functional repercussions. This could be explained by the size of the distal centralizer used, which was at least 1 mm smaller in diameter than the size of the last rasp of the centralizer. Our findings indicated that despite mal-alignment, there was adequate metaphyseal fill and the HA coating promoted bone ingrowth, with no clinical significance [Figure 4]. On the other hand, the present stem revised because of subsidence was attributed to a technical error in establishing a metaphyseal fill, besides its varus position. In effect, Capello38 highlighted the technical skill needed for the use of this hip system.

Bone atrophy of the proximal end of the femur following total hip arthroplasty is widely established.41 We recorded shielding-related bone atrophy in 75% of the hips. This adaptive process was observed during the first few years of follow-up, yet there was no progression thereafter. Cancellous condensation indicating osseointegration in stems, stable by bone ingrowth, mainly appeared in Gruen zones 2 and 6, which explained the high proportion of proximal bone atrophy. This behavior was typical of stems with a greater surface area for bone ingrowth, such as, extensively porous stems. Thus, we could infer that OMNIFLEX stems became more distally integrated in the host tissues despite their porous circumferential surface coated with HA, which circumscribed the metaphyseal zone, probably due mainly to the grit-blasted finish of the middle third of the stem. Takahashi35 reported proximal bone atrophy in 65% of the second generation and 68% of the third generation stems, in patients of a mean age of 64 years. The mean age of our patients was above 75 years. Younger, more active patients with better bone quality develop less atrophy due to stress shielding. The three stems showing stable fibrous ingrowth exhibited grade I atrophy, which could be attributed to the fibrous fixation. We noted atrophy below the lesser trochanter without diaphysis involvement (third grade) in 16% of the cases. This incidence was lower than that reported by others.4142 The OMNIFLEX stem was shaped like a double wedge and was less stiff, explaining its behavior. Notwithstanding, according to the other authors,43 we were aware of the radiographic limitations of assessing bone mineral density loss, and concur with Glassman44 who stresses on the importance of the use of DEXA (dual energy X-ray absorptiometry) in this field.

Polyethylene wear was not greater in the revised stem because of mechanical loosening. As shown in other studies, neither was this PE wear greater in femurs showing atrophy due to stress shielding,45 precluding its relation with osteolysis induced by debris.

The weakness of our study is that the number of dropouts is high (11%), but that is in accordance with others studies.5 This is difficult to avoid in this elderly population who frequently have associated comorbidities. Another weakness could be the number of patients. However, despite these limitations arising from the number of patients we reviewed during the exact period of time of one year, a reliable database, medical records, and radiographs were collected to make a unique set of data that allowed us to address the survival of such an uncemented femoral stem, in different classifications of femoral bone, in elderly patients. The main strength of this study is the age of the patients, 75 years of age and older, and the follow-up mean of 10,4 years, in comparison with other reports [Table 3]. The findings of this study indicate that the third generation OMNIFLEX modular stem achieves adequate biological fixation, with favorable clinical–radiographic results obtained in the long-term in this cohort of 52 prostheses, in elderly patients. However, we feel the objective for which this stem was created has not been fulfilled, given the high proportion of stress shielding-induced bone atrophy observed, higher than the reported one, probably due to the patient age. We do not think this problem should affect stem stability, although it could be a concern in the long term. Based on our experience, primary arthroplasties in our unit are essentially performed using a cementless femoral stem, regardless of the patient's age and femoral canal type.