1 Introduction

It was the research of Sir John Charnley that revolutionized the introduction of total hip arthroplasty for the treatment of patients with severe hip arthritis.¹ As a private practice surgeon, he undertook the responsibility of reporting failures, successes, and the outcomes of his patients. It was his example that inspired a private practice surgeon to carefully follow his patients over the span of ten years with the intention of mitigating historical problems such as thigh pain, distal bone hypertrophy, and proximal femoral stress shielding.

Charnley's use of bone cement to mechanically stabilize femoral stems in place had drawbacks.² The reported considerable drawbacks of cement have been the exothermic effect on bone, material creep, issues with consistent interdigitation with host bone, and potential fragmentation of the cement.² Cemented total hip arthroplasty (THA) has seen a decline in use as surgeons have increasingly shifted towards cementless fixation to avoid these issues.

The choice between cemented and cementless femoral stems in hip arthroplasty continues to remain a topic of discussion. A recent national database study found similar rates of periprosthetic fracture, but higher rates of infection, aseptic loosening, venous thromboembolism, and aseptic revision when using cemented fixation.³ Early cementless stems presented several challenges, including a high incidence of thigh pain associated with femoral component design, stress shielding, and difficulty with revisions for extensively coated stems.^4–6 To address these known issues, newer cementless stem designs had been introduced; these updated stems often featured a proximal porous coating that promoted bone growth and secure attachment, along with a more pronounced taper that enhanced load distribution and reduced thigh pain, thereby minimizing the risk of stress shielding.^7,8 Additionally, the materials used in cementless stems have evolved. Titanium alloys, which better mimic the mechanical properties of cortical bone, have gradually replaced the more rigid cobalt-chrome alloys. Titanium can reduce the likelihood of stress shielding and improve the integration of the implant with the bone,^9,10 making cementless designs an attractive option for surgeons.

The femoral design by Zweymuller demonstrated excellent clinical follow-up outcomes due to its double wedge design, which promoted mechanical interlocking and rotational stability of the femoral stem without porous coating.¹¹ Based on the success of the Zweymuller-style prosthesis, the Klassic HD® stem (Total Joint Orthopedics, Inc., Salt Lake City, Utah) was introduced to allow for improved fixation through a titanium porous-coating, with a grit blasted mid-portion for enhanced attachment, while leaving the bottom portion polished to prevent preferential distal engagement, hypertrophy, and thigh pain (Fig. 1). Its proximal section was coated with Ti-Coat®, a titanium layer with over 60 % porosity, enhancing bone integration. The femoral stems were available in both standard and high-offset versions. This non-modular stem offered neck shaft angles of 121° and 131°. The collarless design was to allow for full press-fit implantation, simplifying both insertion and removal. The stem came in a range of sizes (1–9) and lengths (110–150 mm), with a standard 12/14 neck taper that was compatible with 32 mm or 36 mm cobalt chrome femoral heads, and is currently available with ceramic heads.

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Fig. 1 Image of the Klassic® Hip System showing the three levels of stem surface variance including the proximal Ti-Coat®, grit-blasted mid-portion, and polished distal end. The porous material of the acetabular cup is also coated with Ti-Coat® and features multiple screw holes with 30° sweep and 1.5 mm of press-fit due to its rim-flared design. The polyethylene liner sits flush within the cup. This image was provided with the permission of TJO.

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The Klassic HD® cup is a thin-walled Titanium-alloy acetabular cup with 3.5 mm thickness and 1.5 mm of press-fit at the rim, featuring the same porous coating as the Klassic HD® stem. This Ti-Coat® material had previously demonstrated excellent bone ingrowth as measured in a clinical retrieval.¹² Cups were available in sizes from 44 mm to 64 mm. The cup provided multiple screw holes with 30° sweep which were optimally positioned to provide flexibility in screw placement. The acetabular inserts provided zero protrusio helping to easily match head center, and were manufactured from sequentially crosslinked and annealed polyethylene. Each cup size was matched to a single insert size, which optimized polyethylene thickness and head size. The femoral heads were available in 28, 32, and 36 mm diameters. At least two cancellous bone screws were used to enhance fixation in all patients.

There were two hypotheses tested in this study: one was that the hip system would maintain a physiologic bone response as measured by radiographic assessment, while preventing distal impingement and thigh pain. Secondly, the results would meet clinical standards as measured by Harris Hip Score (HHS).

2 Methods

3 Results

At the 5-year time point, data was available for 85 of the original 100 patient cohort after accounting for 6 deaths and 9 patients lost to follow-up (Table 1).¹⁵ At the 10-year time point, 27 patients had passed away for reasons unrelated to their total hip arthroplasty and 6 were lost to follow-up, leaving 10-year data available for 67 (91.7 %) of the remaining 73 patients.

All patients underwent cementless stem and cup fixation. Average age at the time of surgery was 61.4 ± 10.3 years. Average time to follow-up was 10.7 ± 0.6 years. Of the remaining patients, 23 were male and 44 were female. Mean BMI at the time of surgery was 30.7 ± 5.0 kg/m². The rate of post-operative trochanteric bursitis was 8.2 % in the 5-year cohort and 2.9 % in the 10-year. All instances of trochanteric bursitis were treated successfully with physical therapy and anti-inflammatory medication.

HHS, a clinical assessment tool measuring function, pain, and range of motion with a maximum score of 100, was collected; higher scores correlated with better outcomes. At the 5-year time point, the cohort reported a mean score of 89.4 ± 8.3. At the 10-year time point, the mean score was 90.1 ± 8.7. There was no significant difference in scores between these time points (p = 0.40).

There were six complications requiring reoperation in this study (Table 2). Of these six complications, three were reoperations for debridement of infected hematomas. These hematomas occurred prior to the lead author's use of tranexamic acid and were managed with irrigation and debridement, implant retention, and insertion of an antibiotic-leeching cement disc. There was no persistence of infection after this treatment. The mean time from the index procedure to the reoperation in these patients with infected hematomas was 14.6 ± 1.5 days.

In one patient, a revision was performed for pain related to persistent psoas tendinitis, which was managed with iliopsoas tendon release and repositioning of the acetabular cup to a less horizontal position. One patient experienced recurrent instability several years after their index procedure due to Brooker class 2 heterotopic bone formation and was managed via conversion to a constrained liner with debridement of heterotopic bone; no further complications were identified after their revision procedure. Mean time to follow-up for revisions unrelated to infected hematoma was 6.4 ± 3.5 years. 10-year survivorship of the femoral stem was 100 % and acetabular cup was 99 %. One patient sustained a fall near the 10-year post-operative mark resulting in a Vancouver C periprosthetic fracture, which was treated with open reduction internal fixation with retention of the femoral stem.¹⁵ None of the complications were associated with femoral stem design.

There were three patients that reported symptoms consistent with iliopsoas tendonitis or impingement, which was managed successfully with conservative measures. Hip dislocation was reported in one patient and was managed with closed reduction; there were no further episodes of instability noted in this patient. There were no instances of thigh pain reported in the study group.

In the lead author's prior 5-year follow-up study, the acetabular cup from one of the revisions was extracted from a 54-year-old woman after being implanted for 28 months.¹² The analysis revealed up to 54 % bone integration within the porous coating's available pore spaces. Osteocytes within the lacunae were visualized adjacent to the porous coating suggesting healthy, viable bone.

Analysis of the most recent radiographs revealed no evidence of stem subsidence, distal impingement, or adverse bone response (Fig. 2). Minimal osteolytic changes were observed in Gruen zone 2 in two patients, Gruen zone 6 in one patient, Charnley zone 2 in three patients, and Charnley zone 1 in two patients; these changes were without clinical consequence. Average follow-up time for radiographic review was 43.2 ± 37.4 months (range 1.2–120.5 months). The physiological bone response was within the parameters of mechanical load to maintain attachment between bone and implant.

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Fig. 2 AP and lateral radiographs of a patient at the 10-year post-operative mark who underwent bilateral total hip arthroplasties using the Klassic® Hip System. Imaging demonstrated no stem subsidence or distal impingement, maintenance of acetabular cup position, and minimal osteolytic changes.

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4 Discussion

Similar to Charnley, a critical review was required in order to advocate for the continued use of this femoral design in clinical practice. At the 10-year mark, this cementless total hip arthroplasty system demonstrated excellent outcomes, with 99 % cup and 100 % stem survivorship, high HHS, low revision rates, minimal osteolytic changes, and absence of thigh pain. This newer generation of stem may have a unique advantage over earlier generations because of its more anatomic shape, as well as its variance of material properties allowing for optimal in-growth, on-growth, and reduced stress shielding. The titanium more closely resembled the rigidity of cortical bone and may have contributed to its biocompatibility. In addition to this, the absence of a fully-coated porous stem may allow for easier removal. For example, it is known that distal in-growth may require more extensive measures for removal, such as an extended trochanteric osteotomy.¹⁶

In this study, no subsidence or periprosthetic fracture was noted. The author's previous study demonstrated excellent ingrowth of the available porous spaces in the Ti-Coat® material (up to 54 %) 28-months after surgery.¹² This supported the use of the coating clinically. This same material was used in the proximal portion of the Klassic HD® stem, and further research is needed to characterize the degree of in-growth into the proximal portion of the stem.

While 3 cases of infected hematomas were noted in our study, this complication rate has dramatically declined with the use of Tranexamic Acid (TXA).¹⁷ Our cohort had three patients (3 %) with iliopsoas tendinitis managed non-operatively—this is comparable to another study with 1602 posterior hips that demonstrated a rate of 3.3 %.¹⁸ This could be related to cup positioning, cup sizing, or limb lengthening.¹⁹ Dislocations/instability requiring reoperation in our study was 2 %. A large meta-analysis of 125 studies looking at over 5 million hip replacements found the dislocation rate to be between 2 and 10 % depending on the study.²⁰ Our incidence of greater trochanteric bursitis after total hip was 2.9–8.2 %, which is within the range seen in other studies.^21,22

While this study provides valuable insights into the long-term clinical outcomes of a cementless total hip arthroplasty system, several limitations should be acknowledged. First, the study was conducted in a single-surgeon setting, which may limit the generalizability of the findings to broader surgical practices with varying techniques and expertise. Second, the sample size was relatively small due to patients lost to death or follow-up. Another limitation is the reliance on radiographic analysis to assess bone response and implant stability, which, while valuable, may not fully capture microscopic changes in bone integration. Finally, complications such as iliopsoas tendinitis were reported, but further investigation is needed to determine their potential relationship to implant design, surgical technique, or other patient-specific variables. Future studies with larger, multi-center cohorts and comparative analyses would strengthen the findings and provide a more comprehensive evaluation of this hip arthroplasty system.

5 Conclusion

This study demonstrated the exceptional performance of this cementless hip system, showcasing excellent HHS, low revision rates, 99 % cup and 100 % stem survivorship, and no observed thigh pain, reaffirming its effectiveness and reliability. The two hypotheses were supported in that patients maintained the clinical standard of high HHS scores over the 10-year period, and that the design of the femoral stem demonstrated a physiological bone response, with the avoidance of distal impingement and thigh pain. The cup maintained appropriate positioning with minimal radiolucency over the same period of time.

CRediT authorship contribution statement

Alexander Nielsen: Formal analysis, Investigation, Visualization, Validation, Writing – original draft. Jesua Law: Investigation, Visualization, Validation, Writing – original draft. Matthew Parkin: Investigation, Validation, Writing – original draft. Aaron Hofmann: Conceptualization, Methodology, Supervision, Validation, Writing – review & editing.

Consent

General Consent for Research was received from received from the patient's within this cohort.

Ethical statement

This research was conducted in accordance with the code of ethics of the World Medical Association.

Funding statement

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.