Virtual range of motion is a safe and acceptable alternative to intra-operative manual range of motion testing in direct anterior robotic total hip arthroplasty

Abstract

Introduction

In robotic total hip arthroplasty (THA), virtual range of motion (VROM) modeling allows the surgeon to account for spinopelvic motion and create an impingement-free range of motion that is patient-specific. The primary purpose of this study was to evaluate the risk of dislocation in patients undergoing direct anterior THA using VROM rather than manual ROM trialing.

Methods

Prospectively collected data was reviewed retrospectively of all consecutive anterior THAs performed by a single fellowship-trained surgeon. No patients were excluded from the study. VROM identified bone and implant impingement, which was recorded in degrees of hip external rotation(ER) at 0° of hip extension (standing), and in degrees of hip internal rotation(IR) at 90° of hip flexion (sitting). No patients had manual ROM trialing performed. Dislocation events were recorded during the first 3 months of follow-up. 362 patients, with a mean age of 67 and mean BMI of 28.8, were included.

Results

This cohort, including 154 patients (42.5%) with abnormal spinopelvic motion, demonstrated zero dislocations using VROM. The average ER impingement occurred at 60.9° ER (range 20-90°), and 50.8° IR (range 25-90°). Patients with ER impingement <55° had significantly less acetabular anteversion (16.4° ±3.3°, p < 0.001). Compared to the entire cohort, anteversion of the acetabulum was also decreased in the stuck-sitting subgroup (17.4° ±3.3°, p < 0.001) and increased in the stuck standing subgroup (20.5° ±3.6°, p < 0.001). ER impingement was a stronger predictor of acetabular anteversion than spinopelvic motion category (r = 0.458). Patients with ER impingement <45° (6.4%) or IR impingement <35° (6.6%) were “early impingers”.

Conclusion

In this cohort of anterior THA patients with a high proportion of abnormal spinopelvic motion, a technique utilizing only VROM produced no dislocations. An impingement-free zone of 45° ER standing and 35° IR sitting is recommended.

1. Introduction

Prosthetic hip dislocation is a frequent complication after total hip arthroplasty (THA), with a risk of instability ranging from 0.17 to 1.74% within two years, resulting in the need for revision surgery, leading to a poorer quality of life and higher medical costs. Several operative factors can contribute to hip instability, including preservation of the static and dynamic hip stabilizers, spinopelvic motion, restoration of leg length and offset, and accurate acetabular cup positioning. Component positioning is crucial to implant function and survival. Malpositioning can increase wear rates, shorten the longevity of the implant, and can also lead to dislocation, impingement, or fracture.¹ Advances in surgical technology have allowed for the assessment of these factors and incorporation of them into achieving optimal implant positioning using robotic arm assistance.² Accuracy of implant placement, as assessed radiographically, is a key point in the analysis of robotic THA outcomes. To achieve this, surgeons have historically used pre-operatively determined “safe zones”, as defined by either Lewinnek et al. (inclination 30–50°; anteversion 5–25°) or Callanan et al. (inclination 30–45°; anteversion 5–25°).¹ Emara et al. described in their systematic review that robotic THA had superior acetabular cup positioning within both Lewinnek's and Callanan's safe zones in the 10 studies they reviewed.³ In another study comparing robotic and conventional THA, Bendich et al. showed that robotic THA had significantly decreased rates of dislocation at 1 year when compared to manual THA from a posterior approach.⁴

One of the many operative factors to consider in THA is spinopelvic motion, which is defined as a complex chain of movements between the spine, pelvis, and hips to accommodate for postural change from standing to sitting. As we transition from standing to sitting, the sacrum moves posteriorly and there is a loss of lumbar lordosis, leading to an increase in acetabular anteversion. A stiff lumbar spine, for example in the setting of spinal fusion, impacts this kinetic chain and reduces the posterior tilt and therefore results in a functional loss of acetabular anteversion. Patients with reduced lumbar movement and fixed spinopelvic alignment from standing to sitting have been shown to be at higher risk of dislocation.⁵

When discussing spinopelvic motion, three measurements are commonly used to define spinopelvic parameters: pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS). Pelvic tilt is the angle created by a line running from the midpoint of the S1 endplate to the center of the femoral heads and the vertical axis. Pelvic incidence is an anatomic parameter of constant value that is unique to each patient and independent of the position of the pelvis. It is defined as the angle between the perpendicular line to the S1 endplate at its midpoint and the line connecting this point to the center of the femoral heads. PI is the sum of PT and SS. SS is defined as the angle between the tangential line at the upper endplate of S1 and the horizontal. It is a dynamic parameter which allows the surgeon to be able to quantify and assess spinopelvic motion through postural changes (Fig. 1).²

Fig. 1.

Lateral x-ray of the pelvis demonstrating the relationshio between sacral slope, pelvic tilt, and pelvic incidence.

The MAKO Robotic Arm Interactive Orthopedic (RIO) system (Stryker, Mahwah, NJ) utilizes a virtual range of motion (VROM) tool that offers real-time feedback, allowing the surgeon to visualize the effect of altering implant position, leg-length and offset on bone-on-bone and implant-on-bone impingement. It allows the surgeon to input previously measured spinopelvic parameters (PT or SS) into the software to account for spinopelvic motion during functional hip position modeling. Using the robotic software, the surgeon can preoperatively evaluate the planned hip arthroplasty component positioning by determining maximum impingement-free hip rotation in both flexion and extension. This feature enables the ability to virtually identify dynamic impingement and acquire instant feedback of adjusting or changing component positioning, a technique now coined “functional cup positioning.” Using this technology, there have been no previous reports defining the VROM “safe zone”, or “impingement-free zone” that should be used as a target for implant positioning. We present a series of consecutive patients that have undergone a direct anterior THA using the MAKO 4.0 VROM software to recommend a safe “impingement-free zone” for hip rotation in both flexion and extension. Additionally, we propose that, because of the ability to incorporate spinopelvic motion, routine manual intra-operative hip stability testing by the surgeon for anterior total hip arthroplasty is no longer necessary and may be safely substituted now with functional virtual ROM testing using robotic software that accounts for the unique spinopelvic motion of each individual patient.

2. Methods

Following Institutional Review Board approval, a retrospective review was conducted on 362 consecutive THAs performed by a single, fellowship trained adult reconstruction surgeon at one academic institution utilizing the direct anterior approach between April 2021 through April 2023. The inclusion criteria were patients undergoing a primary THA for the diagnosis of osteoarthritis, inflammatory arthritis, post-traumatic arthritis, or avascular necrosis. No patients were excluded from the study.

All patients received an AP Pelvis as well as lateral pelvis sitting and standing radiographs for the purpose of preoperative templating and to measure SS in both the sitting and standing positions. All patients underwent a preoperative CT scan of the pelvis in accordance with MAKO protocol. Sitting and standing SS values were input into the software. Default implant positioning was used (acetabulum: 40° inclination, 20° anteversion) to initially perform VROM on the software prior to surgery. All THA were performed using a direct anterior approach using a PURIST Leg Positioning System (Innovative Orthopedic Technologies, Winnie, TX).

Robotic software was used to perform VROM to identify bone and implant impingement, which was recorded by convention in degrees of hip internal rotation (IR) at 90° of hip flexion (sitting), as well as in degrees of hip external rotation (ER) at 0° of hip extension (standing). Adjustments were then made to the position of the acetabular component, leg-length and offset based on the degrees of hip rotation at which impingement occurred during VROM, to maximize the impingement-free arc of motion for each patient. During surgery robotic arm assistance was used to perform reaming in the desired plane with the utilization of stereotactic boundaries. The acetabular cup was also positioned with the assistance of the robotic arm and the haptic tunnel that maintains the planned orientation during implantation. In this prospective cohort study, all patients received a cementless femoral stem (Actis, Depuy, Warsaw, IN) and a peripheral self-locking porous acetabular shell (Trident PSL shell; Stryker, Mahwah, NJ). No patients had manual ROM testing during their THA procedure, relying solely on VROM functional hip testing. Dislocation events were recorded during the first 3 months of follow-up.

3. Data analysis

The demographic characteristics of the cohort were presented using mean and standard deviation and frequency as appropriate. Patients were categorized into specific groups based on their sacral slope (SS) measurements in both standing and sitting positions: “stiff spine” if the SS change from standing to sitting was less than 10°, “stuck standing” if SS was greater than 30° in both positions, and “stuck sitting” if SS was less than 30° in both positions. The percentages of patients falling into different ranges of ER and IR impingement were reported. A comparison of acetabular anteversion was conducted between patients with ER impingement ≤55° and those without, using the Mann-Whitney U test. Linear association between acetabular anteversion and ER was investigated using Pearson correlations. Additionally, multilinear regression analysis was used to investigate the relationship between anteversion and ER while controlling for other factors significantly correlated with ER. The mean change in length and offset was reported for the entire cohort. All statistical analyses were performed using IBM SPSS 26.0, and statistical significance was set at a p-value of 0.05.

4. Results

Table 1 outlines the demographics of the patients included in the study. Of the 362 patients, 166 were male with an average age of 65.4 and an average BMI of 29.94 kg/m² and 196 were female with an average age of 68.4 and an average BMI of 27.81 kg/m². Of the 362 hips, 144 were left sided and 218 were right sided. When evaluating the sacral slope, the average SS standing was 39.71° ± 8.19° and average SS sitting was 20.22° ± 9.45°. With this data, we calculated 67 (18.5%) patients with a “stiff spine”, 47 (13%) patients were “stuck standing”, and 40 (11%) were “stuck sitting”. In total, 42.5% of patients in this cohort exhibited abnormal preoperative spinopelvic motion. Utilizing only VROM and eliminating intra-operative manual ROM testing in this cohort, there were no patients who experienced a hip dislocation during the follow-up period. Utilizing the robotic software, the entire cohort average acetabular inclination was 42.2° (range 37-45°) and the average anteversion was 18.99° (range 10-29°). Anteversion of the acetabulum was significantly different between the stuck-sitting cohort (17.4° ± 3.3°) and stuck standing cohort (20.5° ± 3.6°) (p < 0.001). With regards to final reduction measurements at the end of the procedure, the mean change in leg length compared to preoperative was 5.69 ± 2.9 mm and the mean change in hip offset was 0.71 ± 2.96 mm (Table 2).

Table 1.

Demographics of patient population.

	N	Mean Age (years)	SD	Mean BMI (kg/m2)	SD
Males	166	65.39	9.09	29.94	4.24
Females	196	68.4	9.36	27.81	4.95

Table 2.

Summary of acetabular component position.

	Mean	SD
Inclination	42.22°	1.32°
Anteversion	18.99°	3.28°
Change in Length	5.69 mm	2.93 mm
Change in Offset	0.71 mm	2.96 mm

During VROM, the average ER in extension that was able to be achieved before impingement was 60.9° (range 20-90°), and the average IR in flexion that was able to be achieved was 50.84° (range 25-90°). ER between 45° and 75° combined with IR between 40° and 65° captured 81.2% of patients. “Early impingers” included 6.4% of patients with ER impingement <45°, and 6.6% of patients with IR impingement <35°. In all, 14.4% of patients had ER impingement ≤55°, and those patients had significantly reduced acetabular anteversion (16.4° ± 3.3°, p < 0.001).

ER impingement correlated moderately with acetabular anteversion and was a stronger predictor of acetabular anteversion than spinopelvic motion category (r = 0.458, p < 0.001). Multivariate analysis demonstrated a significant association between ER impingement and acetabular anteversion while controlling for IR, femoral head size, cup size and SS standing (r² = 0.393, p < 0.001). There was no correlation between early ER impingement and femoral head size (p = 0.42).

5. Case examples

Here we present 2 case examples showing how functional hip positioning can change as a result of VROM impingement testing. The first case is of a 60-year-old male undergoing a left THA for advanced osteoarthritis. Pre-operative lateral pelvic radiographs showed his SS at 44° standing and 8° sitting, which were input into the VROM software. Fig. 2 shows the pre-operative radiograph demonstrating that the left hip is 3 mm shorter than the right hip with 8 mm of increased offset. Surgical plan included lengthening the hip by 6 mm and reducing offset by 2 mm. With the SS parameters inputted into the software, the hip is then taken through a VROM to evaluate for impingement. The hip encounters both implant and bony impingement at 80° of ER in extension (Fig. 3A and B) and implant and bony impingement at 60° of IR in flexion (Fig. 4A and B). The starting point for our cup inclination and version in the supine position were 40 and 20°, respectively. Because the impingement-free arc exceeded the safe limits of 45° extension-ER and 35° flexion-IR, it was not necessary to alter acetabular anteversion. The native femoral version was 5° retroversion, and the planned stem version was 5° retroversion. This resulted in a planned combined version of 15°. Fig. 5 shows the final position of the acetabular component while Fig. 6 shows the final reconstruction after reduction has been performed, demonstrating that the left side had been lengthened 6 mm (3 mm lengthened compared to the opposite side) and the offset reduced by 3 mm (5 mm increased compared to the opposite side).

Fig. 2.

Case 1: Pre-operative radiograph of the pelvis, showing the left hip to be 3 mm shorter with 8 mm of increased combined offset compared to the contralateral side.

Fig. 3.

A and 3B. VROM demonstration of bony and implant impingement in 0° of extension and 80° of external rotation.

Fig. 4.

A and 4B. VROM demonstration of bony and implant impingement in 90° of flexion and 60° of internal rotation.

Fig. 5.

Final position of the cup after VROM has been performed to maximize internal and external rotation in flexion and extension, respectively. The final position was determined to be 43° of inclination and 20° of anteversion.

Fig. 6.

Final reconstruction after hip reduction showing the left hip with 6 mm of increased length and 3 mm of decreased offset compared to the pre-operative state.

The second case is a 66-year-old male undergoing a right THA for advanced osteoarthritis. Pre-operative lateral pelvic radiographs showed his SS at 18° standing and 9° sitting, which were input into the VROM software. Fig. 7 shows the pre-operative radiograph demonstrating that the right hip is 2 mm longer than the left hip with 1 mm of increased offset. With the SS parameters inputted into the software, the hip is then taken through a VROM to evaluate for impingement. The starting point for our cup inclination and version in the supine position were 43 and 20°, respectively. However, at those parameters, the hip encounters both implant and bony impingement at 50° of ER in extension (Fig. 8A and B) and implant and bony impingement at 55° of IR in flexion (Fig. 9A and B). To increase the impingement-free ER, the anteversion of the cup was ultimately adjusted to 13°. This allowed the ER to increase from 50° to 65° while only decreasing the IR from 55° to 50° (Fig. 9C and D 9E and 9F, respectively), which both still exceed the safe impingement-free limits of 45° extension-ER and 35° flexion-IR. The native stem version was 10° anteversion, and the planned stem version was 10° anteversion. Fig. 10A shows the final position of the acetabular component while Fig. 10B shows the final reconstruction after reduction has been performed, demonstrating that the right side had been lengthened 4 mm (7 mm lengthened compared to the opposite side) and the offset increased by 0 mm (1 mm increased compared to the opposite side). This example highlights that with an anterior approach, impingement-free extension-ER is favored over impingement-free flexion-IR. Notably, despite reducing acetabular anteversion, the component remains covered by native bone anterior inferior to avoid the risk of psoas impingement (Fig. 10A).

Fig. 7.

Case 2: Pre-operative radiograph of the pelvis, showing the right hip to be 2 mm shorter with 1 mm of increased combined offset compared to the contralateral side.

Fig. 8.

A and 8B. VROM demonstration of bony (osteophyte) and implant impingement in 0° of extension and 50° of external rotation with the cup positioned at 43° of inclination and 20° of anteversion.

Fig. 9A and B.

VROM demonstration of bony and implant impingement in 90° of flexion and 55° of internal rotation. with the cup positioned at 43° of inclination and 20° of anteversion.

Fig. 9C and D.

VROM demonstration of bony impingement in 0° of extension and 65° of external rotation with the cup positioned at 43° of inclination and 13° of anteversion.

Fig. 9E and F.

VROM demonstration of implant impingement in 90° of flexion and 50° of internal rotation with the cup positioned at 43° of inclination and 13° of anteversion.

Fig. 10A.

Final position of the cup after VROM has been performed to maximize impingement-free external rotation. The final position was determined to be 43° of inclination and 13° of anteversion.

Fig. 10B.

Final reconstruction after reduction showing the right hip with 4 mm of increased length and no change in offset compared to the pre-operative plan.

6. Discussion

The primary purpose of this study was to report the dislocation rate on a consecutive series of direct anterior total hip arthroplasty patients using VROM rather than manual intra-operative ROM testing, and to secondarily recommend an “impingement-free zone” to target when performing this procedure. There is sparse literature on the defining boundaries of VROM and what degree of hip rotation at impingement can be considered safe and acceptable in flexion and extension. In their review of robotic THA, Ogilvie et al. used the following parameters: deep flexion at 110° flexion and 40° internal rotation; extension was evaluated at 25° extension and 15° external rotation.⁶ However, they do not report on their outcomes or dislocation rates following robotic THA. Similarly, Hadley et al. performed a clinical and radiographic comparison of convention and robotic THA. They found that patients in the robotic THA cohort compared to the conventional THA cohort had significantly higher Western Ontario and McMaster Universities Arthritis Index (WOMAC) and Harris Hip Scores at final follow up. However, there was no significant differences between the two cohorts regarding cup inclination, hip length difference, hip length discrepancy, and global offset difference, and did not outline any specific VROM parameters.⁷

Intraoperative examination of the hip gives a limited assessment of hip stability as it does not account for dynamic spinopelvic motion. Currently, there is no standardization as to the examination of the hip to determine appropriate stability. Takao et al. found in a study of 21 surgeons in varying levels of training, there were considerable inter-examiner differences in the range of forces generated by the shuck test. The strength of traction forces and flexion angles significantly influenced the distance of displacement of prosthetic heads.⁸ Thus, the task of standardizing the manual intra-operative assessment of the hip is practically impossible. We propose, that because VROM factors in spinopelvic motion, that it is a more inclusive assessment of hip stability than intra-operative manual testing by a surgeon. It is a dynamic assessment rather than a static assessment based off one position of the patient's pelvis during surgery.

Although VROM considers bony and implant impingement, one must also consider the soft tissues around the hip joint, with particular reference to the operative approach. The safe impingement-free arc may be different for anterior and posterior approach, as each has different soft-tissue (capsule and muscular) checkreins to hip rotation, which can contribute to risk of directional dislocation. It is the opinion of the authors that with the direct anterior approach, as long as the flexion-IR impingement free limit exceeds the patients preoperative native hip flexion-IR by at least 30°, the patient is unlikely to functionally impinge after surgery and therefore not at risk for posterior dislocation. This is purported to be due to the persistence of the posterior capsule and external rotators creating a checkrein to hip internal rotation and preventing posterior instability. Similarly, the opposite may be true of anterior instability utilizing a posterior approach, which could be a topic for future research.

There are limitations to this study. This study was performed by a single surgeon at a single institution using a single approach, so the generalizability of the data is limited. This study also requires the use of additional x-rays as well as the need for robotics and specialized software that may not be available to all surgeons and facilities. Additionally, this is a retrospective study without a comparison group to be able to compare those hips that underwent THA prior to the use of VROM to those that underwent THA in this study with VROM. Nonetheless, given these limitations, this study provides a solid foundation for future studies to explore the use of VROM utilizing different surgical approaches as well as following these patients for longer to determine if there is a difference in the long-term outcomes of patients, including late instability, between those hips using VROM and those without.

7. Conclusions

In this consecutive cohort of 362 anterior THA patients with a high percentage of abnormal spinopelvic motion, a technique utilizing VROM produced no dislocations and supplanted the need for intraoperative manual ROM testing. An impingement-free zone of 45° ER standing and 35° IR sitting results in no risk of dislocation and is recommended as the “safe impingement-free arc” when using an anterior approach. It is believed that this arc is approach-specific and will be different for posterior approach THA. Early impingers in both ER and IR can be done safely using VROM with no risk of dislocation due to the ability to predict impingement and modify implant positioning to protect against dislocation. It is recommended to reduce acetabular component anteversion in patients with ER impingement less than 55°. It is the authors’ opinion that the small subgroup of anterior THA patients with very early ER impingement (less than 40°) often require significant reduction in acetabular component anteversion to prevent anterior hip instability because they may be at higher risk for anterior dislocation (although not shown in this series), and surgical approach should be considered carefully in these patients if decreasing acetabular anteversion compromises anterior component bone coverage due to the risk of iliopsoas impingement. In those rare cases a modular or cemented femoral component to alter femoral version, or a posterior approach may be preferred.

Funding/sponsorship

None.

Consent

No consent needed.

Ethical approval

This study received IRB approval from the Feinstein Institutes for Medical Research, Northwell Health (IRB #: 23-0361).

Funding

No funding was received for this study.

CRediT authorship contribution statement

Angelo Mannino: Conceptualization, Methodology, Data curation, Writing – original draft, Writing- Reviewing and Editing. Keith R. Reinhardt: Visualization, Investigation, Supervision, Writing- Reviewing and Editing.

Declaration of competing interests

The authors declare the following conflicts: Keith Reinhardt received financial support from Stryker Consultant outside the submitted work.