Abstract
Background
In the United States, cementless fixation is the gold standard for elective total hip arthroplasty (THA). Many modern cementless stem designs have re-introduced collared stem options in recent years which have demonstrated a lower risk of fracture. Minimal studies, however, outline radiographic performance of this novel stem design. As such, the primary purpose of this single-center study was to determine the radiographic performance, including defining patterns of radiographic incorporation and remodeling, associated with this novel, single stem design.
Methods
Data within the institutional data repository was queried for patients who underwent a primary or conversion THA between January 1st, 2016 and July 31st, 2022. Patients were included in the study if they were 18 years of age or older and had a minimum of a one-year follow-up visit. Patients were excluded if they did not have a radiograph at the one-year follow-up or if the stem was placed in a revision setting. Continuous data were reported as means and standard deviations (± SD), and categorical data were reported as number of cases (n) and percentages (%).
Results
A total of 592 encounters (562 patients) were included in the final analyses. At the one-year postoperative visit, no stems met the criteria for radiographic loosening, 502 (85.2%) patients had distinct radiographic osseointegration of their stem as defined by at least one radiographic spot weld. There was an 18.7% incidence of calcar-collar gaps on initial radiographs and 66.7% of these filled in by one-year. The intraoperative fracture rate was 0.7% without any cases of secondary stem revision or loosening and only 0.8% of stems showed subsidence (i.e., all less than 5 mm) without loosening or revision. Thigh pain within the first year was reported in 1.7% of patients. The all-cause stem revision at one-year was 0.2%.
Conclusion
This study demonstrated excellent rates of healing of this novel stem design. Additionally, this novel stem was associated with low rates of periprosthetic fracture, stem revision, and thigh pain.
Level of Evidence: IV
Keywords: total hip arthroplasty, collared femoral stem, uncemented femoral stem
Introduction
In the United States, cementless fixation is currently the gold standard for elective total hip arthroplasty (THA). Between 2012 and 2019, nearly 96% of all primary THAs utilized a cementless fixation, including more specifically, a femoral component.1 Stability of uncemented implants depends on the initial fit and stability of the implant within the bone. The long-term stability depends on the biological anchoring of the implant to the bone or osseointegration.1-3
Cementless femoral fixation has continued to evolved over the last two decades with changes in stem design, including the length and morphology of the stem, and bone preparation techniques.4,5 Modern stems have shortened and often curved designs to aid insertion through minimally invasive approaches and allow for bone preservation and more proximal osseous loading. Three dimensional metaphyseal “fit-and-fill” stem designs have recently become more popular and are thought to facilitate a more stable and more proximal fixation. This stem design has also demonstrated a lower incidence of periprosthetic fractures compared to flat, single taper, wedge stems.5,6 In addition, bone preparation techniques have evolved with some implant companies utilizing a hybrid broaching system consisting of both extraction and compaction with a single broach. Many modern cementless stem designs have re-introduced collared stem options in recent years to confer axial and rotational stability while reducing fracture risk.5,7-12
The ACTIS® stem (DePuy Synthes Orthopaedics, Warsaw, IN) was introduced in 2016 and combines many of these modern adaptations into a novel design; a triple-tapered (coronal, sagittal, and axial planes) stem designed for metaphyseal fit and fill, with or without a fully coated collar. This shortened stem morphology was developed for patients with various femoral anatomy, and the preparation utilizes hybrid broach technique with compaction and extraction. The entire stem is hydroxyapatite coated and the proximal half of the stem including the collar has a beaded ingrowth surface. The relatively new design has been associated with a lower risk of early fractures, revisions, decreased radiographic findings of migration, and comparable patient-reported outcomes to other older uncemented stems.13-15 In addition, the American Joint Replacement Registry (AJRR) report from 2022 demonstrates the lowest cumulative percent revision at one- and three-years postoperatively using the ACTIS® stem compared to other stems.16 More recently in the 2023 AJRR report, the ACTIS® stem was identified to have the lowest cumulative revision percentage among other active stems (Odd Ratio: 0.70; CIs: 0.80, 0.98).17
To date, few studies have outlined radiographic performance of this novel stem design or patterns of bony incorporation or remodeling. As such, the primary purpose of this single-center study was to determine the rate of osseointegration of the stem at one-year follow-up. Our secondary purpose of this study was to determine the rate, timing, and patterns of radiographic incorporation and remodeling associated with this single stem design. Our tertiary aim was to determine the rate and type of stem-related complications during the first year postoperatively.
Methods
After institutional review board approval, a retrospective review was performed using the institution’s integrated data repository (IDR) and electronic medical record (EMR) system. The IDR is considered a large-scale database that collects patient information across the medical center through the EMR (Epic Systems, Madison, WI). Chart review was performed to identify any data the IDR was unable to provide. Data within the repository was queried for patients who underwent a primary or conversion THA (i.e., CPT codes 27130, 27132) with the ACTIS® stem between January 1st, 2016 and July 31st, 2022. Patients were included in the study if they were 18 years of age or older and had a minimum of a one-year follow-up visit. Patients were excluded if they did not have a radiograph at the one-year follow-up visit or if the stem was placed in a revision setting. Patient characteristics (i.e., age, sex, race, ethnicity), Dorr classification18 and operative data were collected.
Study Objectives
First, the primary objective of this study was to determine the rate of osseointegration of the ACTIS® stem at one-year postoperatively. Digital radiographs were assessed for osseointegration by a single reviewer at four time points (i.e., preoperatively, and six-weeks, six-months, and one-year postoperatively). The single reviewer had five years of experience as an orthopedic surgical resident and was currently completing a one-year fellowship in adult reconstruction. To standardize the review for osseointegration, a standardized review protocol was outlined for consistency. This standardized review protocol was created and reviewed with five fellowship-trained arthroplasty physicians. Gruen Zones were evaluated for spot-welding, lucency, and formation of a neocortex.19 Stems were classified as well incorporated or loose based on these findings and the Engh classification.20 Femoral stems were defined as loose if any radiolucent lines >2 mm, progressive radiolucency surrounding the stem, subsidence > 5 mm, or component migration (varus or valgus). Stems were defined as well incorporated, or osseointegrated, if there was a presence of at least one spot weld in the seven Gruen Zones and the absence of the aforementioned radiographic signs of loosening.20
The secondary objective was to determine the patterns of radiographic healing of this stem. All radiographs were analyzed for patterns of radiographic healing utilizing the Gruen Zones, including radiolucency, calcar gap and healing.20 Each Gruen Zone was analyzed at all time points for every stem and radiographic changes were reported.
Lastly, the tertiary objective was to determine the rate of complications associated with the ACTIS® stem. All radiographs were evaluated for stem-related complications, which included subsidence, stress shielding, and periprosthetic fractures. Additionally, chart review was performed preoperatively and at the one-year follow-up visit to determine the rate of thigh pain. The patient charts containing the phrase “thigh pain” were individually reviewed. Patients with postoperative thigh pain were included and those with thigh pain secondary to radiculopathy were not included.
Data Analysis
Statistical analyses were conducted using SPSS Version 28 (IBM SPSS inc., Chicago, IL, USA). Continuous data (i.e., age, body mass index [BMI], length of stay [LOS]) were reported as means and standard deviations (± SD). Categorical data (e.g., sex, laterality, approach) were reported as number of cases (n) and percentages (%).
Results
A total of 562 patients were identified for this study through the IDR data query. Of those, 30 patients underwent a bilateral procedure capturing a total of 592 encounters which were included in the final data set. The average age of patients was 64.83 ± 11.53 years (range = 18-92) with an average BMI of 29.59 ± 6.53 (range = 9.50-52.40) (Table 1). The average LOS was 1.58 ± 1.70 days (range = 0-17.07). Patients were mostly female (57.1%), non-Hispanic (92.9%), and White (83.6%) (Table 1). Mean time between postoperative visits were as follows: 6.11 ± 1.96 weeks (range = 1.14 – 16.14) for the six-week visit, 24.58 ± 11.29 weeks (range = 5.00 – 64.00) for the six-month visit, and 59.57 ± 32.03 weeks (range = 32.43 – 273.14) for the one-year visit. Most patients underwent a posterior approach THA (79.4%; Table 2) and had a Dorr B femoral classification (81.3%; Table 2). Indications for surgery included osteoarthritis (97.8%; N = 779), femoral head osteonecrosis (26.5%; N = 157), hip dysplasia (7.9%; N = 47), and femoral neck fracture (8.6%; N = 51; Table 2).
Table 1.
Patient Demographics
| Frequency (%) (N = 562) | |||
| Sex | |||
| Male | 241 (42.9) | ||
| Female | 321 (57.1) | ||
| Ethnicity | |||
| Hispanic | 22 (3.9) | ||
| Non-Hispanic | 522 (92.9) | ||
| Unknown | 18 (3.2) | ||
| Race | |||
| Asian | 4 (0.7) | ||
| Black | 57 (10.1) | ||
| Hispanic | 1 (0.2) | ||
| Multiracial | 1 (0.2) | ||
| Other | 13 (2.3) | ||
| White | 470 (83.6) | ||
| Unknown | 16 (2.8) | ||
| Mean ± SD | Minimum | Maximum | |
| Age | 65.83 ± 11.53 | 18 | 92 |
| BMI | 29.56 ± 6.53 | 9.50 | 52.40 |
| LOS, days | 1.58 ± 1.70 | 0.00 | 17.07 |
Table 2.
Preoperative Patient Characteristics
| Frequency (%)a (N = 592) | |
| Laterality | |
| Left | 271 (45.8) |
| Right | 320 (54.1) |
| Bilateral | 1 (0.2) |
| Approach | |
| Anterior | 121 (20.4) |
| Direct Lateral | 1 (0.2) |
| Posterior | 470 (79.4) |
| Osteoporosis | |
| No | 457 (77.2) |
| Yes | 135 (22.8) |
| Osteoarthritis | |
| No | 13 (2.2) |
| Yes | 579 (97.8) |
| Femoral Head Osteonecrosis | |
| No | 435 (73.5) |
| Yes | 157 (26.5) |
| Femoral Neck Fracture | |
| No | 545 (92.1) |
| Yes | 47 (7.9) |
| Hip Dysplasia | |
| No | 541 (91.4) |
| Yes | 51 (8.6) |
| Dorr Classification | |
| A | 103 (17.4) |
| B | 481 (81.3) |
| C | 7 (1.2) |
aThe sum may not equal 100% because percentages were rounded.
Of the 111 (18.7%) patients who had radiographic evidence of a gap between the calcar and the collar of the prosthesis at the time of surgery, 74 (66.7%) of these patients had osseous integration of this calcar gap by the one-year follow-up visit (Table 3; Figure 1a and 1b). Each of these patients underwent radiographic measurement to confirm stability of the stem without subsidence. At the one-year postoperative visit, no stems met criteria for loosening and 502 (85.2%) patients had distinct radiographic osseointegration of their ACTIS® stem defined by at least one area of radiographic spot welding.
Table 3.
Postoperative Patient Characteristics
| Frequency (%)a (N = 592) | |
| Postoperative Calcar Gap | |
| No | 481 (81.3) |
| Yes | 111 (18.8) |
| Calcar Gap Filled | |
| No | 28 (25.2) |
| Partial | 9 (8.1) |
| Yes | 74 (66.7) |
| Periprosthetic Fracture | |
| No | 584 (98.6) |
| Yes | 8 (1.4) |
| Intraoperative | 4 (50.0) |
| Postoperative | 4 (50.0) |
| Vancouver Classification | |
| Intraoperative | |
| A2 | 1 (100.0) |
| Postoperative | |
| AG | 3 (30.0) |
| B1 | 3 (30.0) |
| B2 | 1 (10.0) |
| C | 3 (30.0) |
| Postoperative Revision | |
| No | 584 (98.5) |
| Yes | 7 (1.2) |
| Reason for Revision | |
| Fracture | 2 (28.6) |
| Instability | 5 (71.4) |
| Revision of ACTIS® Femoral Stem | 0 (0.0) |
| No | 591 (99.8) |
| Yes | 1 (0.2) |
| Thigh Pain within 1-year | |
| No | 582 (98.3) |
| Yes | 10 (1.7) |
| Stem Subsidence within 1-year | |
| No | 587 (99.2) |
| Yes | 5 (0.8) |
aThe sum may not equal 100% because percentages were rounded.
Figure 1A to 1B.
(1A) Calcar Gap evident on immediate postoperative radiograph, (1B) Osseointegration with filling in of the Calcar gap and osseointegration in Gruen zone 7 by one-year postoperatively.
Additionally, the rate of osseointegration at six-weeks and six-months was 1.7% and 54%, respectively (Table 4). At the one-year postoperative visit, 22.6% (N = 133) of patients had radiographic healing in all four proximal Gruen Zones (i.e., zones 1, 2, 6, and 7) (Table 4; Figure 2 and Figure 3). In addition, 60.3% and 60.8% of patients demonstrated osseointegration in Gruen Zone 1 and 7, respectively (Table 5). Greater than one-third of all stems demonstrated formation of neocortex and lucency in Gruen Zones 3, 4, and/or 5 (Table 4; Figure 4 and Figure 5).
Table 4.
Postoperative Rates of Radiograph Healing Characteristics
| 6-weeks postoperative (N = 576) N (%) | 6-months postoperative (N = 215) N (%) | 1-year postoperative (N = 589) N (%) | |
| Radiographic Osseointegration | |||
| No | 566 (98.3) | 99 (46.0) | 87 (14.8) |
| Yes | 10 (1.7) | 116 (54.0) | 502 (85.2) |
| Healing for Gruen Zones 1, 2, 6, 7 | |||
| No | 575 (99.8) | 197 (91.6) | 456 (77.4) |
| Yes | 1 (0.2) | 18 (8.4) | 133 (22.6) |
| Formation of neocortex & Lucency | |||
| Zone 3 | 10 (1.7) | 71 (33.0) | 254 (43.1) |
| Zone 4 | 6 (1.0) | 42 (19.5) | 214 (36.3) |
| Zone 5 | 18 (3.1) | 75 (34.9) | 267 (45.3) |
Figure 2.
ACTIS® Femoral Stem showing osseointegration in Gruen Zones 1, 2, 6 and 7 including integration of the collar. Bony integration is favorable for loading and force transfer through the proximal bone.
Figure 3.
Radiographic spot weld in Gruen Zone 1.
Table 5.
Radiographic Review by Gruen Zone
| 6-weeks postoperative (N = 576) N (%) | 6-months postoperative (N = 215) N (%) | 1-year postoperative (N = 589) N (%) | |
| Zone 1 | |||
| Spot weld | 6 (1.0) | 78 (36.3) | 355 (60.3) |
| Lucency | 23 (4.0) | 9 (4.2) | 14 (2.4) |
| Spot weld & Lucency | 1 (0.2) | 9 (4.2) | 17 (2.9) |
| Zone 2 | |||
| Spot weld | 6 (1.0) | 59 (27.4) | 280 (47.5) |
| Formation of neocortex | 1 (0.2) | 1 (0.5) | 3 (0.5) |
| Lucency | 5 (0.8) | 10 (4.7) | 17 (2.9) |
| Spot weld & Lucency | 0 (0.0) | 6 (2.8) | 19 (3.2) |
| Formation of neocortex & Lucency | 0 (0.0) | 0 (0.0) | 1 (0.2) |
| Zone 3 | |||
| Spot weld | 0 (0.0) | 4 (1.9) | 42 (7.1) |
| Formation of neocortex | 6 (1.0) | 56 (26.1) | 200 (34.0) |
| Lucency | 4 (0.7) | 12 (5.6) | 21 (3.5) |
| Spot weld & Lucency | 0 (0.0) | 0 (0.0) | 2 (0.4) |
| Spot weld & Formation of neocortex | 0 (0.0) | 0 (0.0) | 8 (1.4) |
| Formation of neocortex & Lucency | 0 (0.0) | 3 (1.4) | 33 (5.6) |
| Zone 4 | |||
| Spot weld | 0 (0.0) | 4 (1.9) | 23 (3.9) |
| Formation of neocortex | 6 (1.0) | 32 (14.9) | 186 (31.6) |
| Lucency | 0 (0.0) | 4 (1.9) | 10 (1.7) |
| Formation of neocortex & Lucency | 0 (0.0) | 6 (2.8) | 18 (3.0) |
| Zone 5 | |||
| Spot weld | 0 (0.0) | 5 (2.3) | 42 (7.1) |
| Formation of neocortex | 7 (1.2) | 61 (28.4) | 218 (37.0) |
| Lucency | 8 (1.4) | 10 (4.7) | 21 (3.5) |
| Spot weld & Formation of neocortex | 0 (0.0) | 1 (0.5) | 1 (0.2) |
| Formation of neocortex & Lucency | 1 (0.2) | 4 (1.9) | 28 (4.7) |
| Zone 6 | |||
| Spot weld | 3 (0.5) | 39 (18.1) | 239 (40.6) |
| Formation of neocortex | 1 (0.2) | 6 (2.8) | 17 (2.9) |
| Lucency | 9 (1.5) | 12 (5.6) | 20 (3.4) |
| Spot weld & Lucency | 0 (0.0) | 4 (1.9) | 15 (2.5) |
| Formation of neocortex & Lucency | 0 (0.0) | 0 (0.0) | 2 (0.3) |
| Zone 7 | |||
| Spot weld | 4 (0.7) | 69 (32.1) | 358 (60.8) |
| Formation of neocortex | 0 (0.0) | 0 (0.0) | 1 (0.2) |
| Lucency | 3 (0.5) | 9 (4.2) | 6 (1.0) |
| Formation of neocortex & Lucency | 0 (0.0) | 1 (0.5) | 1 (0.2) |
Figure 4.
Radiographic neocortex formation in Gruen Zones 3 and 4.
Figure 5.
Radiographic lucency in Gruen Zone 4.
A total of 8 (1.4%) patients sustained a periprosthetic fracture, including intraoperatively (N = 4) and postoperatively (N = 4; Table 3). Of the patients with a postoperative periprosthetic fracture, two patients underwent a revision surgery (i.e., open reduction internal fixation [N = 1], femoral component revision [ACTIS® stem; N = 1]), while the remaining two were treated non-operatively. An additional five patients (0.8%) underwent a revision procedure (i.e., acetabular component [n = 4], constrained liner [n = 1]) postoperatively due to instability. The average time to revision for the seven patients was 73.62 ± 85.36 weeks (range = 5.29 - 233.43). There was a total of five stems (0.8%) that subsided at the one-year follow-up visit, and of those, two of these occurred in patients who suffered a periprosthetic fracture. None of the stems subsided greater than 5 mm and all went on to radiographic osseointegration at one year follow-up. Additionally, none of the stems that subsided underwent revision surgery. Lastly, there were 10 (1.7%) patients who still reported thigh pain at the one-year postoperative visit.
Discussion
To our knowledge, this is the first study to evaluate the rate of radiographic healing as well as the pattern of healing of this novel femoral stem design. The ACTIS® stem showed reliable overall radiographic healing rates (85.2%) at one-year postoperatively. Also, there were no stems that were found to be loose at one year follow-up. Additionally, the stems demonstrated a low revision rate when assessed at the one-year postoperative follow-up visit.
We found that the ACTIS® stem has a distinctive pattern of healing within our population. Most modern stems predictably show radiographic healing in zones 2 and 6 but show variable healing in zones 1 and 7.21 In addition, zones 3, 4, and 5 show variable healing depending on stem design. This stem showed a pattern of spot welding in the proximal four Gruen Zones (i.e., zones 1, 2, 6, 7) as well as lack of healing, formation of neocortex, and/or lucency in the distal three Gruen Zones (i.e., zones 3, 4, 5). The combination of predictable proximal healing with the low incidence of distal spot-welding results in a bone preserving, proximal pattern of healing. The radiographic formation of neocortex and lack of spot weld in zones 3, 4 and 5 is beneficial to avoid proximal stress shielding. We believe that this unique pattern of radiographic healing is secondary to the novel design features of the stem in question.
We found a low rate of overall complications in this study with a 0.7% rate of intraoperative fracture, a 0.7% incidence of postoperative fracture, a 1.2% all-cause revision rate, a 0.2% rate of ACTIS® stem revision, and a 1.7% incidence of thigh pain. The incidence of thigh pain with short cementless designs is reported to be up to 24%.22 As such, the current study adds valuable single institution results and specific patterns of radiographic osseointegration to recent registry data demonstrating excellent clinical performance of the ACTIS® femoral stem.16,17
Limitations
There are several limitations within our study that warrant discussion. First, this was a retrospective chart review study with a one-year minimum follow-up visit. As such, our data was queried from our IDR, which could have caused for missed patients for this study. Also, there was no direct comparison group in this study. Additionally, radiographic interpretation can be subjective and may vary between reviewers. To account for this variation, a single reviewer assessed the radiographs at the multiple visits postoperatively. Even though intrarater reliability for this reviewer was not calculated, there was a standardized review protocol in place for consistency. Lastly, our institution’s quality and consistency of the lateral imaging prevented us from including those images in the present study. However historical studies evaluating osseous changes around stems have focused and reported only on zones 1-7.23-26
Conclusion
Overall, this study demonstrated excellent rates of healing of this novel stem design, especially in the proximal four Gruen Zones and the calcar in a proximally loading, bone preserving manner. In addition, this stem was associated with low rates of periprosthetic fracture, stem revision, and thigh pain.
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