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Neocortex Formation in a Tapered Wedge Stem is Not Indicative of Complications or Worse Outcomes

Published:January 31, 2022DOI:https://doi.org/10.1016/j.arth.2022.01.069

      Abstract

      Background

      The formation of sclerotic bone, a neocortex, distally surrounding total hip arthroplasty (THA) stems may commonly be seen on radiographs around proximally porous coated stems but can be confused with loosening. The goal of this study was to determine the prevalence of the neocortex finding and whether it associated with worse outcomes after THA.

      Methods

      A retrospective review of 825 patients with a single tapered wedge stem was performed. Radiographs at 1-year, as well as final follow-up were reviewed for evidence of sclerotic bone (neocortex) surrounding the stem in all 14 Gruen zones. The final attending radiology read of lucency was also recorded. Patients were grouped by the presence of the neocortex. PROMIS Physical Function scores and complications were compared between neocortex groups.

      Results

      The neocortex group had 558 (68%) patients compared to 267 (32%) in the no neocortex group. The most common Gruen zones for evidence of neocortex were 10 (55%), 11 (52%), and 12 (51%). Seven percent of patients had a finding of lucency on radiology read. There was no difference between groups in terms of dislocations (P = .61), infection (P = .79), fracture rates (P = .54), revision surgery (P = .73), and reoperation for any cause (P = .62). PROMIS PF scores were significantly higher in the neocortex group (P < .0001).

      Conclusion

      The presence of a distal neocortex is a common finding on radiographs after THA with this proximally porous-coated tapered wedge stem and does not portend worse outcomes, nor is it a sign of aseptic loosening, increased revision rates, infection, dislocation, or periprosthetic fracture risk.

      Keywords

      Total hip arthroplasty (THA) with cementless tapered wedge titanium stems is known to have excellent outcomes and implant survivability [
      • Bourne R.B.
      • Rorabeck C.H.
      • Patterson J.J.
      • Guerin J.
      Tapered titanium cementless total hip replacements: a 10- to 13-year followup study.
      ,
      • Lachiewicz P.F.
      • Soileau E.S.
      • Bryant P.
      Second-generation proximally coated titanium femoral component: minimum 7-year results.
      ,
      • Froimson M.I.
      • Garino J.
      • Machenaud A.
      • Vidalain J.P.
      Minimum 10-year results of a tapered, titanium, hydroxyapatite-coated hip stem: an independent review.
      ,
      • Marshall A.D.
      • Mokris J.G.
      • Reitman R.D.
      • Dandar A.
      • Mauerhan D.R.
      Cementless titanium tapered-wedge femoral stem: 10- to 15-year follow-up.
      ,
      • McLaughlin J.R.
      • Lee K.R.
      Total hip arthroplasty in young patients. 8- to 13-year results using an uncemented stem.
      ,
      • Meding J.B.
      • Keating E.M.
      • Ritter M.A.
      • Faris P.M.
      • Berend M.E.
      Minimum ten-year follow-up of a straight-stemmed, plasma-sprayed, titanium-alloy, uncemented femoral component in primary total hip arthroplasty.
      ,
      • Parvizi J.
      • Keisu K.S.
      • Hozack W.J.
      • Sharkey P.F.
      • Rothman R.H.
      Primary total hip arthroplasty with an uncemented femoral component: a long-term study of the Taperloc stem.
      ,
      • Streit M.R.
      • Lehner B.
      • Peitgen D.S.
      • Innmann M.M.
      • Omlor G.W.
      • Walker T.
      • et al.
      What is the long-term (27- to 32-year) survivorship of an uncemented tapered titanium femoral component and survival in patients younger than 50 Years?.
      ]. Their slim distal design and porous-coated proximal surface provide excellent osteointegration in the metaphysis and allow their use in a broad range of femoral morphologies and approaches to the hip [
      • Faizan A.
      • Wuestemann T.
      • Nevelos J.
      • Bastian A.C.
      • Collopy D.
      Development and verification of a cementless novel tapered wedge stem for total hip arthroplasty.
      ,
      • McLaughlin J.R.
      • Lee K.R.
      Long-term results of uncemented total hip arthroplasty with the Taperloc femoral component in patients with Dorr type C proximal femoral morphology.
      ]. This permits their use in a diverse population, contributing to their popularity in THA. Paramount to the long-term success of these stems is proximal osteointegration of the implant-bone interface.
      Aseptic loosening of the femoral component is a possible cause of postoperative pain, poorer patient-reported outcomes, and higher rates of revision [
      • Bozic K.J.
      • Kurtz S.M.
      • Lau E.
      • Ong K.
      • Vail T.P.
      • Berry D.J.
      The epidemiology of revision total hip arthroplasty in the United States.
      ,
      • Ulrich S.D.
      • Seyler T.M.
      • Bennett D.
      • Delanois R.E.
      • Saleh K.J.
      • Thongtrangan I.
      • et al.
      Total hip arthroplasties: what are the reasons for revision?.
      ,
      • Brown J.M.
      • Mistry J.B.
      • Cherian J.J.
      • Elmallah R.K.
      • Chughtai M.
      • Harwin S.F.
      • et al.
      Femoral component revision of total hip arthroplasty.
      ,
      • Furnes O.
      • Lie S.A.
      • Espehaug B.
      • Vollset S.E.
      • Engesaeter L.B.
      • Havelin L.I.
      Hip disease and the prognosis of total hip replacements. A review of 53,698 primary total hip replacements reported to the Norwegian Arthroplasty Register 1987-99.
      ]. In the evaluation of possible aseptic loosening, the surgeon must correlate clinical symptoms with radiographic assessment. At follow-up appointments, it is important to scrutinize radiographs for any signs of bony changes around the implants. A common finding of implant loosening is a radiolucency adjacent to the metal prosthesis surrounded by a denser rim of bone, likely representing impacted cancellous bone [
      • Madhu T.S.
      • Akula M.R.
      • Raman R.N.
      • Sharma H.K.
      • Johnson V.G.
      The Birmingham hip resurfacing prosthesis: an independent single surgeon’s experience at 7-year follow-up.
      ,
      • Maathuis P.G.
      • Visser J.D.
      High failure rate of soft-interface stem coating for fixation of femoral endoprostheses.
      ,
      • Jakim I.
      • Barlin C.
      • Sweet M.B.
      Radiological signs of loosening of the femoral stem in cementless total hip arthroplasty.
      ]. In other areas of orthopedics, however, a phenomenon of sclerotic bone formation, or a neocortex, surrounding a smooth metal implant that is not associated with loosening has been described [
      • Papadakis S.A.
      • Segkos D.
      • Katsiva V.
      • Panagiota P.
      • Balanika A.
      • Stavrianos V.
      • et al.
      Intramedullary bone formation after intramedullary nailing.
      ]. Interestingly, the phenomenon of a neocortex around has not been previously described around total hip arthroplasty implants.
      Due to the importance of identifying implants that may be at risk for loosening or failure, radiologists are often tasked with reading and interpreting images taken following adult reconstructive surgeries. Without clinical symptoms, it may be difficult for a radiologist to determine the true significance of neocortical formation around a femoral prosthesis (Fig. 1). In our own practice, we have found that this may lead them to comment on a lucency surrounding the distal nonporous segment of tapered wedge THA stems and conclude that this finding is concerning for loosening. These comments can cause undue stress on the patient and other treating providers who rely on the radiology reads of images in the era of patient-centered care. Given the lack of other alternative descriptions of the neocortical formation in the existing literature, our group set out to determine if the finding was correlated with any concerning outcomes following THA. Our hypothesis was that the radiographic finding of sclerotic bone, or a neocortex, distally around a tapered wedge THA stem is not indicative of loosening and is not associated with worse outcomes.
      Figure thumbnail gr1
      Fig. 1Neocortex formation seen in Gruen zones 3, 4, 5 on the left and 10, 11, 12 on the right. Blue arrows pointing to the neocortex seen on these radiographs.

      Methods

      After institutional review board approval, a retrospective cohort study was performed over 10 years (2010-2019) using data from an academic tertiary care hospital. All patients that had been recorded in the medial record having had a single design type of a tapered wedge stem (Taperloc; Zimmer Biomet Warsaw, IN) implanted during primary total hip arthroplasty were retrieved.

      Inclusion Criteria

      Patients who underwent a primary total hip arthroplasty using the tapered wedge stem (Taperloc Complete, Zimmer Biomet, Warsaw, IN) were included. All patients that had at least one recorded PROMIS PF score and postoperative radiograph in the postoperative period were included. The surgical approach was left to the discretion of the attending surgeon and included either direct anterior or anterior-based muscle sparing (ABMS) and posterior approach to the hip.

      Exclusion Criteria

      Patients that had a shortened (microplasty) style tapered wedge stem of a similar design, which were implanted, were also excluded due to the shortened distal extent of the nonporous segment of the stem. While we have seen similar findings around similar style implants from other manufacturers, these were not included for the purpose of this study in order to minimize confounding factors. Additionally, patients that did not have a PROMIS PF score or a radiograph at least one year postoperatively were excluded. Last, those patients with radiographs of the operative hip without both an anterior-posterior and lateral view of the entire stem were excluded.

      Patient Characteristics

      The electronic medical record was used to document demographics for each patient included. These variables included age, sex, body mass index (BMI), American Society of Anesthesiologists (ASA) class, and Charlson Comorbidity Index (CCI).

      Follow-up

      Each attending surgeon at the study institution has a standardized follow-up protocol for patients after total hip arthroplasty. Patients are seen in the clinic at 2 weeks and 6 weeks postoperatively. Patients who are meeting all expectations and have progressed to pain free full weight-bearing without assistive devices are scheduled for follow-up at the 1-year postoperative mark. For patients that are not pain free, or still using assistive devices, they are scheduled to follow-up at the 3-month mark as well. Appointments past 1 year postoperatively are encouraged at the 2-, and 5-year time points. Once a patient arrives at the clinic, the front desk staff provides them with an iPad that contains PROMIS questions. Included are the PF subcategory questions needed to provide a patient with their PROMIS PF score. While not required, patients are encouraged to complete the form. Their scores are then recorded in the electronic medical record and able to be accessed at any time point. Complications such as dislocation, infection, and fracture were identified by chart review.

      Radiographs

      Patients’ radiographs were reviewed at the 1-year postoperative and final follow-up when applicable. Follow-up dates were calculated by the difference between the date of radiographs and the surgery date. Those patients that did not have a radiograph within 2 years but did have radiographs further from surgery were included as only having final follow-up radiographs. Those patients with only radiographs in the 1 to 2-year postoperative period were considered to only have 1-year postoperative radiographs. A neocortex was defined as a sclerotic margin appearing on the radiographs adjacent to the hip prosthesis. The presence of such a finding, as well as other radiolucent lines, was recorded for each of the 14 Gruen zones, 7 on the AP and 7 on the lateral. Finally, an attending radiology read of the postoperative radiographs was reviewed to determine whether there was mention of ‘loosening of’ or ‘lucency surrounding’ the femoral prosthesis.

      Statistical Analysis

      Demographics, complications, and patient-reported outcomes were compared between patients where a neocortex formation was noted on their 1-year postoperative radiographs and those where a neocortex formation was not noted using t-tests, chi-squared, and Fisher’s exact tests. The difference between preoperative and postoperative patient-reported outcomes was also compared using paired t-tests between neocortex groups. McNemar’s tests were used to compare the presence of neocortex formations in each Gruen zone between 1-year postoperative radiographs and final follow-up radiographs.

      Results

      There were 2052 patients that were identified to have had a tapered wedge femoral stem placed while undergoing primary total hip arthroplasty. Of those excluded, 612 did not have any PROMIS PF data recorded in the EMR, 348 did not have follow-up past the 1-year postoperative mark, 253 did not have radiographs at or past the 1-year postoperative mark, and 14 had a microplasty stem placed. As such, 825 patients were included in the final analysis, including 701 patients with a 1-year follow-up and 124 patients with follow-up not at the one-year mark but at a further date past 2 years postoperatively (Fig. 2).
      Overall, the average age of the entire cohort was 61.2 years (SD 12.7), with 440 (53.3%) females and 385 (46.7%) males (Table 1). The average BMI was 29.9 (SD 6.7), ASA 2.2 (SD 0.6), and Charlson Comorbidity Index (CCI) 1.4 (SD 1.9).
      Table 1Demographics.
      DemographicOverall

      N = 825
      No Neocortex

      N = 267
      Neocortex

      N = 558
      P Value
      Mean (Std)Mean (Std)Mean (Std)
      Age61.2 (12.7)62.5 (13.9)60.7 (12.1).0625
      BMI29.9 (6.6)31.1 (6.8)29.3 (6.5).0004
      ASA2.2 (0.6)2.4 (0.6)2.1 (0.6)<.0001
      Charlson Comorbidity Index1.4 (1.9)1.8 (2.1)1.2 (1.7)<.0001
      n (%)n (%)n (%)P Value
      Gender
       Female440 (53.3%)127 (47.6)313 (56.1).0216
       Male385 (46.7%)140 (52.4)245 (43.9)
      Smoking status
       Never smoker561 (38.8%)170 (64.9)391 (70.7).0939
       Ever smoker254 (31.2%)92 (35.1)162 (29.3)
      When grouped by presence of neocortex formation, the neocortex group had 558 (68%) patients compared to 267 (32%) in the no neocortex group. There was no difference in age between those with neocortex formation (60.7, SD 12.1) vs those without (62.5, SD 13.9) (P = .06); however, there were more females (56.1% vs 47.6%; P = .02), BMI was lower (29.3, SD 6.5 vs 31.1, SD 6.8, P = .0004), ASA was lower (2.1, SD 0.6 vs 2.4 SD 0.6, P < .0001), and CCI was lower (1.2, SD 1.7 vs 1.8, SD 2.1; P < .0001) in the neocortex group. The most common Gruen zones for evidence of neocortex were 10 (457 patients, 55%), 11 (425, 52%), and 12 (422, 51%) (Fig. 3). The next most common zones were 4 (289, 35%), 5 (150, 18%), and 3 (92, 11%). The least common zones were 2 (0, 0%), 7 (1, 0%), 6 (7, 1%), and 14 (12, 1%). Seven (41/558 patients) percent of patients in the neocortex group had a finding of ‘lucency’ or ‘loosening’ on radiology read.
      Figure thumbnail gr3
      Fig. 3Incidence of Neocortex formation by Gruen zones for all patients.
      For the 340 patients with both 1-year and final follow-up films, Gruen zones 3 (9.7% vs 14.7%, P = .01) and 4 (29.7% vs 35.6%, P = .03) were more likely to be presence on final follow-up; whereas Zone 1 (5.0% vs 2.9%, P = .02) was more likely to resolve (Table 2). No other zones had changes between follow-up. Our cohort had only two cases of aseptic loosening, with an overall survival rate of 99.8% with aseptic loosening as the endpoint (Fig. 4). Interestingly, PROMIS PF scores were statistically significantly higher in the neocortex group (44.8 vs 42.1; P < .0001) (Fig. 5). However, when compared between change in preoperative and postoperative PROMIS PF function scores, there was no difference between groups (P = .175) (Table 3). Furthermore, there was no difference between groups (no neocortex vs neocortex) in terms of dislocation (2.3 vs 2.9, P = .6049), infection (2.3 vs 2.0, P = .7941), fracture (0.4 vs 0.2, P = .5428), revision (2.6 vs 3.1, P = .7341), and reoperation for any cause (4.9 vs 4.1, P = .6231) (Table 4).
      Table 2Comparison of Gruen Zones Between One-Year Postoperative and Final Radiograph Reads.
      Gruen ZoneOne Year Radiograph reads

      N = 340
      Final Radiograph reads

      N = 340
      P value
      N (%)N (%)
      Zone 117 (5.0)10 (2.9).020
      Zone 200
      Zone 333 (9.7)50 (14.7).005
      Zone 4101 (29.7)121 (35.6).025
      Zone 557 (16.8)63 (18.5).330
      Zone 61 (0.3)4 (1.2).083
      Zone 700
      Zone 87 (2.1)5 (1.5).157
      Zone 911 (3.2)14 (4.1).366
      Zone 10196 (57.7)208 (61.2).134
      Zone 11192 (56.5)189 (55.6).729
      Zone 12190 (55.9)190 (55.9)1.000
      Zone 1314 (4.1)16 (4.7).527
      Zone 149 (2.7)7 (2.1).157
      Figure thumbnail gr4
      Fig. 4Kaplan–Meier survival curve for aseptic loosening.
      Figure thumbnail gr5
      Fig. 5Preoperative (A) and postoperative (B) patient-reported outcomes.
      Table 3Comparison of Patient-Reported Outcome Measures Between Patients Who had a Neocortex and Those Who did Not.
      Patient-Reported OutcomeNo Neocortex

      N = 267
      Neocortex

      N = 558
      P value
      Mean (Std)Mean (Std)
      Change from preoperatively to postoperatively
      Physical function7.1 (8.1)8.3 (8.9).175
      Physical health7.2 (9.1)7.0 (8.1).856
      Mental health3.4 (8.0)3.5 (7.4).841
      Global health5.0 (16.7)3.8 (16.0).485
      Pain−28.1 (33.4)−29.0 (30.8).810
      Table 4Comparison of Complications Between Patients Who had a Neocortex and Those Who did Not.
      ComplicationOverall

      N = 825
      No Neocortex

      N = 267
      Neocortex

      N = 558
      P value
      N (%)N (%)N (%)
      Dislocation22 (2.7)6 (2.3)16 (2.9).605
      Infection17 (2.1)6 (2.3)11 (2.0).794
      Fracture2 (0.2)1 (0.4)1 (0.2).543
      Revision24 (2.9)7 (2.6)17 (3.1).734
      Reoperation36 (4.4)13 (4.9)23 (4.1).623

      Discussion

      Aseptic loosening has historically been cited as one of the most common causes for revision THA, cited in current literature as being the cause of 20-50% of revision THAs [
      • Bozic K.J.
      • Kurtz S.M.
      • Lau E.
      • Ong K.
      • Vail T.P.
      • Berry D.J.
      The epidemiology of revision total hip arthroplasty in the United States.
      ,
      • Ulrich S.D.
      • Seyler T.M.
      • Bennett D.
      • Delanois R.E.
      • Saleh K.J.
      • Thongtrangan I.
      • et al.
      Total hip arthroplasties: what are the reasons for revision?.
      ,
      • Brown J.M.
      • Mistry J.B.
      • Cherian J.J.
      • Elmallah R.K.
      • Chughtai M.
      • Harwin S.F.
      • et al.
      Femoral component revision of total hip arthroplasty.
      ,
      • Furnes O.
      • Lie S.A.
      • Espehaug B.
      • Vollset S.E.
      • Engesaeter L.B.
      • Havelin L.I.
      Hip disease and the prognosis of total hip replacements. A review of 53,698 primary total hip replacements reported to the Norwegian Arthroplasty Register 1987-99.
      ]. Bozic et al evaluated 51,000 revision THAs and found that mechanical loosening was the cause of nearly 20% of revisions overall and the cause of 25% of isolated femoral component revisions [
      • Bozic K.J.
      • Kurtz S.M.
      • Lau E.
      • Ong K.
      • Vail T.P.
      • Berry D.J.
      The epidemiology of revision total hip arthroplasty in the United States.
      ]. Ulrich et al evaluated 237 revision THAs and found that aseptic loosening was the cause of failure in 52% of cases but did not differentiate between acetabular or femoral component loosening [
      • Ulrich S.D.
      • Seyler T.M.
      • Bennett D.
      • Delanois R.E.
      • Saleh K.J.
      • Thongtrangan I.
      • et al.
      Total hip arthroplasties: what are the reasons for revision?.
      ]. Given that aseptic loosening of the femur is a common cause of revision THA, it is vital that the orthopedic surgeon and musculoskeletal (MSK) radiologist correctly understand concerning radiographic findings and correlate them with clinical findings associated with aseptic loosening. The findings of radiolucency adjacent to an implant, subsidence of the implant, and clinical symptoms of startup thigh pain and increased thigh, groin, or knee pain with weight-bearing are often used to help diagnose a loose femoral stem. Implant loosening can commonly be seen as a lucency adjacent to the metal prothesis surrounded by a denser rim of bone, likely representing impacted cancellous bone. In this study, we have studied a different radiographic phenomenon, of a neocortex, which has implications that appear to be vastly different than a radiolucency. To our knowledge, there is little research to describe a neocortex around the distal nonporous portion of a proximally porous cementless tapered wedge stem, and our findings show that this radiographic sign does not portend worse outcomes, nor is it a sign of aseptic loosening, increased revision rates, infection, dislocation or periprosthetic fracture risk.
      With the use of this tapered wedge stem design, we found that approximately two-thirds of patients had formed a neocortex around the distal end of the stem. This neocortex was most often seen on the lateral hip radiograph in Gruen zones 10, 11, and 12. Moreover, this finding in those Gruen zones did not change over time. Conversely, the presence of a neocortex was more likely to be present in zones 3 and 4 over time. Given the commonality of this neocortex, it is paramount for the orthopedic surgeon to understand the significance of this finding. We propose a possible explanation for the radiographic finding, which is similar to that seen in other orthopedic implants, and believe this is a phenomenon that occurs when the metaphyseal portion of the stem is well ingrown, and the stiffness mismatch of the implant and bone causes a small area of sclerosis distal to the tip of the stem. A neocortex at the distal end of a taper-wedged stem should not be confused for the pedestal sign, which has previously been described as a sign concerning aseptic loosening in other implant designs [
      • Madhu T.S.
      • Akula M.R.
      • Raman R.N.
      • Sharma H.K.
      • Johnson V.G.
      The Birmingham hip resurfacing prosthesis: an independent single surgeon’s experience at 7-year follow-up.
      ,
      • Maathuis P.G.
      • Visser J.D.
      High failure rate of soft-interface stem coating for fixation of femoral endoprostheses.
      ,
      • Jakim I.
      • Barlin C.
      • Sweet M.B.
      Radiological signs of loosening of the femoral stem in cementless total hip arthroplasty.
      ]. It has previously been shown that tight distal fixation of a tapered wedge stem can decrease osteointegration of the proximal porous-coated portion of the stem [
      • Cooper H.J.
      • Jacob A.P.
      • Rodriguez J.A.
      Distal fixation of proximally coated tapered stems may predispose to a failure of osteointegration.
      ]. If the implant is appropriately sized, it will not pot distally, allowing for excellent press-fit and osteointegration of the metaphyseal portion of the stem. Seeing this neocortex may be indicative that the implant is appropriately sized both proximally, and the smooth nonporous distal segment of the implant is appropriately resting within the diaphyseal region distally.
      In our review of 825 patients who underwent primary THA with a proximally porous uncemented tapered wedge femoral stem, we found similar survivorship compared to what has previously been published. Our cohort had only 2 cases of aseptic loosening leading to revision with a survival rate of 99.8% at 3 years with aseptic loosening as the endpoint. Uncemented tapered wedge femoral stems have previously been reported to have excellent survivorship and patient-reported outcomes [
      • Bourne R.B.
      • Rorabeck C.H.
      • Patterson J.J.
      • Guerin J.
      Tapered titanium cementless total hip replacements: a 10- to 13-year followup study.
      ,
      • Lachiewicz P.F.
      • Soileau E.S.
      • Bryant P.
      Second-generation proximally coated titanium femoral component: minimum 7-year results.
      ,
      • Froimson M.I.
      • Garino J.
      • Machenaud A.
      • Vidalain J.P.
      Minimum 10-year results of a tapered, titanium, hydroxyapatite-coated hip stem: an independent review.
      ,
      • Marshall A.D.
      • Mokris J.G.
      • Reitman R.D.
      • Dandar A.
      • Mauerhan D.R.
      Cementless titanium tapered-wedge femoral stem: 10- to 15-year follow-up.
      ,
      • McLaughlin J.R.
      • Lee K.R.
      Total hip arthroplasty in young patients. 8- to 13-year results using an uncemented stem.
      ,
      • Meding J.B.
      • Keating E.M.
      • Ritter M.A.
      • Faris P.M.
      • Berend M.E.
      Minimum ten-year follow-up of a straight-stemmed, plasma-sprayed, titanium-alloy, uncemented femoral component in primary total hip arthroplasty.
      ,
      • Parvizi J.
      • Keisu K.S.
      • Hozack W.J.
      • Sharkey P.F.
      • Rothman R.H.
      Primary total hip arthroplasty with an uncemented femoral component: a long-term study of the Taperloc stem.
      ,
      • Streit M.R.
      • Lehner B.
      • Peitgen D.S.
      • Innmann M.M.
      • Omlor G.W.
      • Walker T.
      • et al.
      What is the long-term (27- to 32-year) survivorship of an uncemented tapered titanium femoral component and survival in patients younger than 50 Years?.
      ]. Furthermore, some studies have made mention of a finding similar to a neocortex around such femoral stems [
      • McLaughlin J.R.
      • Lee K.R.
      Total hip arthroplasty in young patients. 8- to 13-year results using an uncemented stem.
      ,
      • Parvizi J.
      • Keisu K.S.
      • Hozack W.J.
      • Sharkey P.F.
      • Rothman R.H.
      Primary total hip arthroplasty with an uncemented femoral component: a long-term study of the Taperloc stem.
      ,
      • Rao R.R.
      • Sharkey P.F.
      • Hozack W.J.
      • Eng K.
      • Rothman R.H.
      Immediate weightbearing after uncemented total hip arthroplasty.
      ,
      • McLaughlin J.R.
      • Lee K.R.
      Cementless total hip replacement using second-generation components: a 12- to 16-year follow-up.
      ]. However, these studies found a much lower rate of neocortex formation, as well as, at different Gruen zones, than the current study. Last, these studies did not focus solely on the impact of neocortex formation. In a long-term study of hips evaluated at an average of 28 years, tapered wedge stems had a 94% survivorship with aseptic loosening as the endpoint [
      • Streit M.R.
      • Lehner B.
      • Peitgen D.S.
      • Innmann M.M.
      • Omlor G.W.
      • Walker T.
      • et al.
      What is the long-term (27- to 32-year) survivorship of an uncemented tapered titanium femoral component and survival in patients younger than 50 Years?.
      ]. Early aseptic failures were most often due to undersized femoral stems, and late failures were most often due to periprosthetic fracture [
      • Streit M.R.
      • Lehner B.
      • Peitgen D.S.
      • Innmann M.M.
      • Omlor G.W.
      • Walker T.
      • et al.
      What is the long-term (27- to 32-year) survivorship of an uncemented tapered titanium femoral component and survival in patients younger than 50 Years?.
      ]. Bourne et al reported on 307 hips with 10-13 year follow-up with no revision THAs performed for aseptic femoral loosening [
      • Bourne R.B.
      • Rorabeck C.H.
      • Patterson J.J.
      • Guerin J.
      Tapered titanium cementless total hip replacements: a 10- to 13-year followup study.
      ]. Lachiewicz et al followed 55 hips for a minimum of 7 years and found that no femoral component had to be revised for any reason [
      • Lachiewicz P.F.
      • Soileau E.S.
      • Bryant P.
      Second-generation proximally coated titanium femoral component: minimum 7-year results.
      ]. McClaughlin et al followed 91 patients for a minimum of 10 years and showed that no femoral component needed to be revised for aseptic loosening. Last, these types of findings are not unique to the hip, with similar radiograph lines not affecting the outcomes of patients with revision total knee arthroplasties [
      • Sah A.P.
      • Shukla S.
      • Della Valle C.J.
      • Rosenberg A.G.
      • Paprosky W.G.
      Modified hybrid stem fixation in revision TKA is durable at 2 to 10 years.
      ].
      Not only did we find no correlation of the finding of a neocortex with aseptic loosening, but we also found that the formation of a neocortex was not indicative of any other worse outcomes. In fact, we interestingly showed a statistically significantly higher PROM score in the neocortex group. There was no difference between groups in the incidence of dislocations, fractures, infections, and need for revision surgery (Table 1). However, of those patients with a neocortex, seven percent had a finding of lucency on MSK radiology read. Therefore, this finding is not only important for surgeons to understand but also for MSK radiologists to be aware that this is not likely a concerning finding.
      This study is not without limitations. The retrospective nature of the study inherently creates the risk for selection bias. While patients had at least one year of follow-up, there is a risk that patients were lost to follow-up that had an adverse outcome. However, the revision rates and rates of adverse outcomes were similar for other reported tapered wedge stems [
      • Bourne R.B.
      • Rorabeck C.H.
      • Patterson J.J.
      • Guerin J.
      Tapered titanium cementless total hip replacements: a 10- to 13-year followup study.
      ,
      • Lachiewicz P.F.
      • Soileau E.S.
      • Bryant P.
      Second-generation proximally coated titanium femoral component: minimum 7-year results.
      ,
      • Froimson M.I.
      • Garino J.
      • Machenaud A.
      • Vidalain J.P.
      Minimum 10-year results of a tapered, titanium, hydroxyapatite-coated hip stem: an independent review.
      ,
      • Marshall A.D.
      • Mokris J.G.
      • Reitman R.D.
      • Dandar A.
      • Mauerhan D.R.
      Cementless titanium tapered-wedge femoral stem: 10- to 15-year follow-up.
      ,
      • McLaughlin J.R.
      • Lee K.R.
      Total hip arthroplasty in young patients. 8- to 13-year results using an uncemented stem.
      ,
      • Meding J.B.
      • Keating E.M.
      • Ritter M.A.
      • Faris P.M.
      • Berend M.E.
      Minimum ten-year follow-up of a straight-stemmed, plasma-sprayed, titanium-alloy, uncemented femoral component in primary total hip arthroplasty.
      ,
      • Parvizi J.
      • Keisu K.S.
      • Hozack W.J.
      • Sharkey P.F.
      • Rothman R.H.
      Primary total hip arthroplasty with an uncemented femoral component: a long-term study of the Taperloc stem.
      ,
      • Streit M.R.
      • Lehner B.
      • Peitgen D.S.
      • Innmann M.M.
      • Omlor G.W.
      • Walker T.
      • et al.
      What is the long-term (27- to 32-year) survivorship of an uncemented tapered titanium femoral component and survival in patients younger than 50 Years?.
      ]. Additionally, all our patients underwent surgery with two orthopedic surgeons at a single academic medical center with a single proximally coated cementless tapered wedged stem and may decrease the applicability of our findings to a wider audience. The appearance of this finding on other tapered wedge stems cannot be confidently extrapolated as being benign without further studies in patients with those implants. Finally, we did not collect the incidence of thigh pain in our dataset and are unable to state whether the finding of a neocortex is associated with thigh pain.
      Future studies could be warranted to further evaluate the finding of a neocortex in THA. We feel that a neocortex could be seen in other proximally coated cementless tapered wedged stems or possibly in other metaphyseal fitting stems. More studies analyzing this finding in other stems would add to the literature and help orthopedic surgeons and MSK radiologists better understand the radiographic significance of a neocortex surrounding the distal end of the stem in proximally porous-coated stem designs.

      Conclusion

      The presence of a distal neocortex is a common finding on follow-up radiographs after THA with this proximally porous-coated tapered wedge stem and does not portend worse outcomes, nor is it a sign of aseptic loosening, increased revision rates, infection, dislocation, or periprosthetic fracture risk. Surgeons and MSK radiologists alike should be aware of this common radiographic finding when using tapered wedge-designed stems.

      Appendix A. Supplementary Data

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