Patient-reported and radiographic outcomes of a porous-coated acetabular cup in robotic assisted total hip arthroplasty at 2-year follow up

Abstracts

Introduction

Several implant manufacturers have developed ultra-porous metal substrate acetabular components recently. Despite this, data on clinical and radiographic outcomes remain limited. Our study evaluated postoperative patient-reported outcome measures (PROMs) and radiographic analyses in patients fitted with a novel acetabular porous-coated component.

Methods

A total of 152 consecutive patients underwent a total hip arthroplasty by a single orthopaedic surgeon. All patients underwent surgery utilizing the same CT-scan based robotic-assisted device with the same porous cementless acetabular shell. They received standardized postoperative physical therapy, rehabilitation, and pain protocols. Preoperatively, first postoperative visit, 6-months, 1-year, and 2-years, patients were evaluated based on Western Ontario and McMaster Universities Arthritis Index (WOMAC) pain, physical function, and total scores; 2) Patient-Reported Outcomes Measurement Information System (PROMIS)-10 physical and mental scores; 3) Hip Disability and Osteoarthritis Outcome Score (HOOS)-Jr scores; as well as 4) acetabular component positions and 5) evidence of acetabular component loosening.

Results

Significant improvements were observed by 6 months in WOMAC pain, physical function, and total scores (p < 0.05), maintained at 1 and 2 years. PROMIS-10 physical scores also improved significantly from preoperative to 6 months postoperative and remained so at 1 and 2 years postoperative (p < 0.05). No significant changes were found in PROMIS-10 mental scores. HOOS-Jr scores significantly improved from preoperative to 6 months postoperative and remained so through 2 years (p < 0.05). At 6 months, slight changes were noted in abduction angle and horizontal and vertical offset. Radiolucencies, initially found in 3 shells, reduced to 1 shell with 2 new radiolucencies by 6 months, and remained stable with no subsequent operative interventions. At 1 year and 2 years, no radiographic abnormalities were noted, including complete resolution of prior radiolucencies as well as stable components.

Conclusion

This porous cementless acetabular shell, implanted with CT-scan-based robotic-assisted techniques, demonstrated excellent postoperative PROMs at 2 years. Stable radiolucencies suggest good component stability. The early stable clinical and radiographic results suggest promising long-term outcomes with this device.

Level of evidence

III (retrospective cohort study)

1. Introduction

For total hip arthroplasty (THA) in the past 15 years, multiple manufacturers have developed ultra-porous acetabular components.^1,2 The theoretical benefits of these implants include a lower modulus of elasticity (which can reduce stress shielding), high porosity (to potentially improve bone ingrowth), and a high coefficient of friction (to facilitate fixation). Put some references here There have been many studies delineating excellent survivorship of these components, for example, one report finding aseptic survivorship of 99.6 %,³ and another study with 100 % survivorship at 2- to 4-year follow-up)⁴

There have also been some potential concerns with these acetabular prostheses. For example, in a case series by Ulrich et al., two patients experienced screw/shell interface failure in one of these acetabular systems.⁵ Another study found an alarmingly high rate of progressive acetabular sclerotic lines and/or radiolucencies.⁶ Fortunately, even with these radiographic concerns, there was still an acetabular survivorship of 98.2 %.⁶

In addition, there is also a paucity of data highlighting the outcomes and accuracy of cup placement after preoperative CT based, robotic-arm assisted THA. Therefore, the purpose of this study was to evaluate postoperative patient-reported outcome measures (PROMS) and to perform a radiographic analysis following the placement of a new ultra-porous cementless acetabular cup system with robotic assistance. Specifically, we assessed: 1) WOMAC pain, physical function, and total scores; 2) Patient-Reported Outcomes Measurement Information System (PROMIS)-10 physical and mental scores; 3) Hip Disability and Osteoarthritis Outcome Score (HOOS)-Jr scores; as well as 4) acetabular component positions and 5) evidence of acetabular component loosening.

2. Methods

2.1. Institutional Review Board approval

After obtaining Institutional Review Board approval, a retrospective analysis was performed to identify all patients who had previously undergone a primary total hip arthroplasty (THA) at our institution with a new second generation acetabular system between February 21, 2017, and December 6, 2018.

2.2. Patient selection

A total of 152 patients underwent THA by a single orthopaedic arthroplasty surgeon. Their mean age of patients was 64 years (range, 33–86 years) and they had a mean body mass index (BMI) of 35 (range, 28 to 42). There were 74 men (49 %) and 78 women (51 %). Patient demographic information can be noted in Table 1.

Table 1.

Patient demographic breakdown.

Patient Demographic Breakdown
N=152
Mean age in years (Range)	64 (33–86)
Mean BMI (kg/m2) (Range)	35 (28–42)
Sex
Men (%)	74 (49)
Women (%)	78 (51)
Laterality
Right (%)	88 (58)
Left (%)	64 (42)

BMI = Body mass index.

2.3. Surgical technique and prostheses used

The second-generation acetabular implant utilized was the Trident II Acetabular Cup System (Stryker Corporation, Kalamazoo, Michigan). This was made by an additive manufacturing process as a cementless design with hydroxyapatite coating on a Titanium metal substrate with an average pore size of 434 μm with 76 % surface porosity (See Fig. 1, Fig. 2).⁷ The design was based on Stryker Orthopedics Modeling and Analytics (SOMA) technology, from a database of 520+ Computed Tomography (CT)-scans.^8,9

Fig. 1.

Trident II titanium clusterhole.

Fig. 2.

Trident II titanium solidback.

The MAKO robotic platform (Mako Surgical Corp. [Stryker], Fort Lauderdale, Florida) was utilized in all cases. This platform is based on a patient's preoperative CT-scan which creates a virtual rendering of the bone and joint that can be manipulated in real-time in the operating room to account for patient specific anatomy. The real-time virtual adjustments in the operating room allow the surgeon to determine the most optimal implant positioning and placement for each patient. The software also takes into consideration limb lengths and spinal pathologies (i.e., account for sacral slope, pelvic incidence, and pelvic tilt). All THAs were performed utilizing a standard anterior hip approach. Postoperatively, they underwent standardized physical therapy, rehabilitation, and pain protocols.

2.4. Patient reported outcome measures

Three separate patient reported outcomes measures were assessed. These PROMS included: 1) WOMAC; 2) PROMIS; and HOOS-Jr scores. The WOMAC score is an assessment of osteoarthritis, so higher scores are indicative of increased pain and decreased function. The WOMAC survey was separated based on a subset of pain and physical function as well as an aggregated total score. The PROMIS score was subdivided into physical and mental scores, with higher numbers indicating a more positive response. The HOOS-Jr is scaled to 100 points, with a higher score indicating greater postoperative THA patient satisfaction.

PROMS assessments were taken preoperatively for a baseline value, and then subsequently at approximately 6-months, 1-year, and 2-years postoperatively.

2.5. Radiographic assessments

Radiographic assessments were made by the primary surgeon and reviewed by the entire operative team. Specific measurements taken included acetabular component: 1) abduction angle; 2) horizontal offset; 3) vertical offset; as well as changes in these 3 measurements over time. Additionally, acetabular component loosening, determined by surrounding radiolucencies were evaluated. Radiographic assessments were taken at the first postoperative visit (between 2 and 6 weeks postoperative) and at 6-months postoperative.

2.6. Data analyses

Descriptive statistics were utilized to compare qualitative data points, such as patient demographics. For PROMS, a one-way ANOVA with post hoc Tukey tests were performed with 95 % confidence. Differences in radiographic measurements were taken as absolute values.

3. Results

Patient-Reported Outcomes Measure (PROMS)- Western Ontario and McMaster Universities Arthritis Index (WOMAC):

Compared to preoperative scores, all Western Ontario and McMaster Universities Arthritis Index scores were significantly improved by 6-months and maintained to be significantly improved at 1- and 2-years (p < 0.05). Specifically, 2-year WOMAC pain (mean ± Standard Deviation (SD): 1 ± 2 points), physical function (mean ± SD: 2 ± 3 points), and total scores (mean ± SD: 3 ± 5 points), were significantly improved than preoperative WOMAC pain (mean ± SD: 10 ± 3 points), physical function (mean ± SD: 14 ± 5 points), and total scores (mean ± SD: 24 ± 8 points).

A further breakdown of scores can be noted in Table 2.

Table 2.

Preoperative, 6-month, 1-year, and 2-year Western Ontario and McMaster Universities Arthritis Index (WOMAC) scores.

WOMAC	Pre-op		6-month		1-year		2-year
	Mean ± SD	Range	Mean ± SD	Range	Mean ± SD	Range	Mean ± SD	Range
Pain	10 ± 3	0 to 20	1 ± 2	0 to 8	1 ± 2	0 to 7	1 ± 2	0 to 11
Physical Function	14 ± 5	1 to 28	2 ± 3	0 to 10	2 ± 2	0 to 9	2 ± 3	0 to 14
Total	24 ± 8	3 to 48	3 ± 4	0 to 16	3 ± 4	0 to 16	3 ± 5	0 to 24

SD=Standard deviation.

Patient-Reported Outcomes Measure (PROMS) – Patient-Reported Outcomes Measurement Information System (PROMIS)-10:

PROMIS 10 physical scores were significantly improved from preoperative to 6-months postoperative and maintained to be significantly improved at 1- and 2-years postoperative (p < 0.05). Specifically, preoperative physical scores (mean ± SD: 44 ± 8 points) improved at 6-months (mean ± SD: 54 ± 7 points), 1-year (mean ± SD: 53 ± 7 points), and 2-years (mean ± SD: 52 ± 7 points) (See Table 3).

Table 3.

Preoperative, 6-month, 1-year, and 2-year Patient-Reported Outcomes Measurement Information System (PROMIS)-10 physical and mental scores.

PROMIS 10	Pre-op		6-month		1-year		2-year
	Mean ± SD	Range	Mean ± SD	Range	Mean ± SD	Range	Mean ± SD	Range
Physical	44 ± 8	20 to 62	54 ± 7	32 to 68	53 ± 7	30 to 68	52 ± 7	32 to 68
Mental	54 ± 7	31 to 68	56 ± 7	39 to 68	54 ± 7	28 to 68	54 ± 7	36 to 68

SD=Standard Deviation.

PROMIS 10 mental scores showed no statistical differences between preoperative (mean ± SD: 54 ± 7 points), 6-months (mean ± SD: 56 ± 7 points), 1-year (mean ± SD: 54 ± 7 points), and 2-years postoperative (mean ± SD: 54 ± 7 points) (See Table 3).

Patient-Reported Outcomes Measure (PROMS) – Hip Disability and Osteoarthritis Outcome Score (HOOS)-Jr:

HOOS-Jr scores were significantly improved from preoperative to 6-months postoperative and maintained to be significantly improved at 1- and 2-years postoperative (p < 0.05). Specifically, scores preoperatively (mean ± SD: 50 ± 13 points) improved at 6-months (mean ± SD: 90 ± 10 points), 1-year (mean ± SD: 90 ± 13 points), and at 2-years postoperatively (mean ± SD: 91 ± 12 points) (See Table 4).

Table 4.

Preoperative, 6-month, 1-year, and 2-year Hip Disability and Osteoarthritis Outcome Score (HOOS)-Jr.

PROM Measure	Pre-op		6-month		1-year		2-year
	Mean ± SD	Range	Mean ± SD	Range	Mean ± SD	Range	Mean ± SD	Range
HOOS-Jr	50 ± 13	0 to 100	90 ± 10	70 to 100	90 ± 13	40 to 100	91 ± 12	53 to 100

SD=Standard Deviation.

3.1. Radiographic assessments – acetabular component measurements

At the first preoperative visit, mean acetabular component abduction angle was 42.9 ± 3.4°, horizontal offset was 27.5 ± 3.5 mm, and vertical offset was 18.0 ± 3.2 mm. At 6-month follow up, the mean change in abduction angle was 1.8 ± 1.7°, horizontal offset was 1.4 ± 1.2, and vertical offset was 1.0 ± 0.90. These minimal changes in measurements from first postoperative to 6-months postoperative visit indicate stable component positions with minimal migration (See Table 5). At 1-year and 2-years, no radiographic abnormalities were noted, including complete resolution of prior radiolucencies as well as stable components.

Table 5.

Radiographic acetabular component measurements.

Cup position	2–6 weeks		6-month
	Mean ± SD	Range	Mean ± SD	Range
Abduction angle (deg)	42.9 ± 3.4	36.0 to 53.0	42.4 ± 3.7	28.0 to 50.0
Horizontal offset (mm)	27.5 ± 3.5	21.4 to 36.0	27.3 ± 3.7	21.8 to 38.6
Vertical offset (mm)	18.0 ± 3.2	11.0 to 28.8	17.5 ± 2.8	11.5 to 25.5
Change in abduction angle (deg)			1.8 ± 1.7	0 to 10
Change in horizontal offset (mm)			1.4 ± 1.2	0 to 7.1
Deg=degrees			1.0 ± .90	0 to 3.6

SD=Standard deviation, mm = millimeters.

3.2. Radiographic assessment – acetabular component loosening

A total of 3 acetabular shells had radiolucencies at the first postoperative visit. These radiolucencies were seen again at 6-months postoperative and did not propagate past 1 mm. One acetabular shell with a 1 mm radiolucency in Zone 1, one shell with a 1 mm radiolucency in Zone 2, and the third shell with a 1 mm radiolucency in Zone 3. At 6-months postoperative, 1 shell had 2 new radiolucencies; 1 mm in zone 1 and 1 mm in zone 2. No radiolucency was progressive. Despite any radiolucencies, all components were determined to be stable. No further interventions, operative or non-operative, were performed based on noted radiolucencies. All patients with radiolucencies had excellent clinical outcomes.

4. Discussion

The new second generation acetabular system has been shown to have excellent post operative patient satisfaction and component placement.10, 11, 12 Generally, data on ultra-porous cups are often reported in separate patient populations and usually with a single PROMS tool.¹³ Additionally, changes in implant measurements have not been well-reported in the literature. The results from this study indicate that a second-generation acetabular system may have the potential to achieve significantly improved satisfaction outcomes based on multiple PROMS as early as six months post-operatively and maintain these outcomes at two years. Additionally, the second-generation acetabular system was noted to maintain stable component placement from the first post operative visit to six months post operative. This finding suggests early adequate bony adherence of the acetabular component potentially.^14,15

The described excellent results concerning the new second generation acetabular system after a two-year mean follow-up are consistent with the current literature. Malahias et al. performed a high-quality analysis of peer-reviewed literature on ultra-porous coated titanium acetabular cups in primary and revision total hip arthroplasty (THA).¹⁶ Sixteen reports were included in this systematic review consisting of 10,886 cases in primary THA. Overall findings of the ultra-porous cup were associated with high survivorship (99.3 %) and minimal rates of aseptic loosening (0.1 %), instability (0.2 %), and periprosthetic joint infection (0.2 %).¹⁶ In addition, 8 of those 16 studies evaluating primary THA assessed clinical and radiographic outcomes.^6,11,13,17, 18, 19, 20, 21 One of them, Sodhi et al., reported on 3-year outcomes in patients who demonstrated excellent aseptic (99.6 %), all-cause (98 %) survivorship, and functional outcomes in a previous, similar iteration of the components studied in this report.²¹ Interestingly, these authors noted three other studies examining the first-generation primary Titanium cup and reported increased rates of radiolucent lines.^6,20,22 While most reports have favorable outcomes, the authors suggested future studies be conducted for the second-generation cup due to concerns of increased radiolucencies in the first-generation cup. Therefore, our study was designed to address these concerns and report the results using this new second generation acetabular system.

Presently, there is a paucity of literature evaluating the new second generation acetabular system. Its use may be necessary as some literature suggests that the first-generation cup might have increased radiolucencies.^14,15 For example, Carli et al. compared the clinical and radiographic results of 109 hips in 95 patients using the first-generation primary cup.⁶ One-year radiographs revealed that 40 % of cups had radiolucent and radiosclerotic lines in at least two zones and 17 % in 3 zones at a minimum 5-year follow-up. The results revealed poor osseointegration with features of fibrous ingrowth and, in some cases, complete failure of osseointegration.⁶ This was the first study to report unfavorable outcomes, but fortunately this was contrasted by the largely successful performance of the first-generation cup in multiple other studies.4-5 references with most close to 98 % survivorships or greater. Our radiographic assessment of the second-generation cup identified three acetabular shells with radiolucencies. Only one shell had radiolucencies in two zones, and two shells had radiolucencies in 1 zone. Fortunately, no progressive radiolucencies were noted in the current study. The methods regarding the radiographic assessment of the acetabular cup are fundamentally identical to the aforementioned study, yet the results at six months already show a difference. This might be due to the controlled network of pores emulating the complex characteristics of cancellous bone, promoting long-term biologic fixation in this second-generation acetabular system.

In this study, the combination of CT based, robotic-arm assisted THA may have played an important role in cup placement and PROMS. Chen et al. performed a review of CT-based robotic-assisted technologies compared to traditional arthroplasties.²³ Of the 63 reports, 21 evaluated THA in studies conducted through a database, national registry, health utility, or comparison method. All off the THA studies demonstrated improved clinical results for CT scan-based robotic arm-assisted hip arthroplasty[23, 24, 25, 26, 27, 28, 29, 30, 31, 32]. These results included lower complication rates, shorter lengths of stay, decreased costs, and improved pain and physical function scores. Four of the studies evaluated PROMS primarily through Harris Hip (HHS) and Forgotten Joint Scores (FJS).29, 30, 31, 32 One of them, Domb et al., reported on five-year outcomes in patients who underwent robotic-assisted THA and demonstrated significantly higher Harris Hip Scores, Forgotten Joint Scores-12, Veterans RAND-12, Physical, and 12-Item Short Form Survey Physical Scores (p < 0.001, p = 0.002, p = 0.002, p = 0.001, respectively) when compared to manual THAs.³² Additionally, five of the 21 reports assessed component positioning and postoperative complications following CT-based robotic-assisted THA.24, 25, 26, 27, 28 One of which, Illgen et al., assessed outcomes at two years.²⁷ They found the rate of acetabular component placement within the Lewinnek safe zone was highest in the CT-based robotic-assisted THA cohort compared to manual THA. Similarly, the other four reports presented analogous findings consistent with improved component positioning with CT-based robotic-assisted THA.²⁷ These studies suggest that the utilization of robotic-arm assistance may have abetted the positive results following the placement of the second-generation new acetabular system.

Other studies have also noted advantages to CT scan-based robotic arm-assisted hip arthroplasty from a health utility perspective. Sarrel et al. evaluated the literature on the cost-effectiveness of CT scan-guided, 3-Dimensional, robotic-arm lower extremity arthroplasty.³³ A total of five assessed articles on hip arthroplasty, four were in favor of CT scan-guided, 3-dimensional, robotic-arm assisted THA. Robotic-assisted surgery resulted in a decreased LOS across all studies, greater in-hospital costs in all studies except one in which in-hospital costs were lower, decreased 90-day EOC costs, lower likelihood of discharge to skilled nursing facility (SNF) or acute rehabilitation, lower post-discharge costs, increased utilization of home health services with lower associated home health costs, and increase in quality-adjusted life-years (QALYs).34, 35, 36, 37, 38 From an economic perspective, implementing the new second-generation acetabular system with robotic assistance may benefit from both a patient and cost savings standpoint.

This study is not without limitations. There was no other acetabular system control group for comparison. We did not include preoperative or postoperative radiographs demonstrating radioluciences but reported complete resolution of prior radiolucencies as well as stable components. Additionally, PROMS results were evaluated up to two years while radiographic measurements were performed out to six months. Furthermore, this study only evaluated a single surgeon's first experiences with the new acetabular system. Future studies should build on this work by incorporating a control cohort as well as a longer duration of patient reported outcomes and radiographic measurements. Nevertheless, this study provides a baseline foundation supporting the use of this operative technology and implant.

5. Conclusion

Despite the tremendous success of total hip arthroplasty, advances in acetabular components have allowed for further improvement in this operation. New iterations of ultra-porous titanium acetabular components have been proven to improve clinical outcomes and patient satisfaction potentially. The results from this study identified encouraging PROMS for the new second generation acetabular system across multiple metrics at two years follow-up. Additionally, the radiographic findings indicated optimal acetabular placement, and low rates of migration or radiolucencies, indicating stable and adequate bone-implant integration. It appears that the robotic assistance may have been synergistic with the use of this second-generation titanium shells would provide the most benefit. Therefore, based on these results, we recommend the implementation of CT based, robotic-arm assisted THA s as well as the second generation acetabular system for patients undergoing THA.

Disclaimers

Funding/Sponsorship: No funding was disclosed by the authors.

Declaration of competing interest

One or more of the authors, with which they are affiliated have received financial payments or other benefits from any commercial entity related to the subject of this article.