1 Introduction

Robotic assisted arthroplasty has seen an increase in popularity among both patients and surgeons in the last decade. ¹ Robotic-assisted total hip arthroplasty (RATHA) requires pelvic array placement that creates potential for pin placement complications including iatrogenic injury to neurovascular structures. One case study describes the loss of a pelvic array pin during RTHA in part due to bone loss which resulted in laparoscopic retrieval. ² However, there is no literature looking into neurovascular injury associated with array pins.

Although there have been studies focused on pin placement for pelvic external fixation, it also is limited and does not account for varying pin placement to facilitate room for instrumentation to perform a robotic hip arthroplasty.³ Manufacture surgical guides for RATHA have not included extensive anatomical references, leaving array placement to surgeon preference and technique, and there is lacking evidence on how to place them. Pin placements in pelvic external fixation often use an open approach and fluoroscopic guidance. In RATHA, pelvic array design and placement are based on a percutaneous approach and palpation of anatomic landmarks highlighting the need for safer and more objective approaches to pin placement. In addition, consideration for the difference among genders in regards to pelvic variations has not been considered in pelvic external fixation nor manufacturer designs. ^4,5

Given the lack of objective data on pin placement and preventive measures to mitigate neurovascular injury in RATHA, we aim to determine 1) caudal angle trajectory of pin insertion furthest away from the neurovascular structures, 2) if there is a difference in distances from pin to neurovascular structure based on gender of patient, and 3) if caudal angle of the array pin is associated with distal portion of the pin being in bone (if distal portion of pin deemed to be in bone, would have better purchase and the trajectory may be considered as being safe even if the pin is deemed to be close to neurovascular structure).

2 Methods

The study included nine MRIs which were void of deformities, arthritic changes and other pathologies. Our study population was composed of 6 females and 3 males from the University of Miami Health Systems’ database. We focused on four anatomical structures: the lateral femoral cutaneous nerve (LFCN), internal iliac artery (IIA), superior gluteal artery (SGA), superior gluteal nerve (SGN).

We recreated the array pin trajectory using the measurement feature on Philips IntelliSpaceRadiology. Zones were created to better determine how close the pin was to the neurovascular structures and were defined within the two-thirds of the pin in the patient (approximately one-third is outside). Of that two-thirds of the pin, four equal zones were determined, from superficial to deep: A1, A2, B1, and B2 (Fig. 1). We defined the AP distance as the distance from pin entry point to the midpoint of every zone and recreated the pin trajectory through both the oblique and caudal trajectories. The oblique angle was defined through the coronal view, modeling the patient anatomy from lateral to medial. The caudal angle was defined through the sagittal view, from anterior superior to posterior inferior approach. In addition, the caudal angle was taken from a transverse plane in the sagittal view. On transverse plane: 35° = 55° on longitudinal plane, 45° = 45° on longitudinal plane, and 55° = 35° on longitudinal plane (Figs. 2 and 3). If the available imaging did not allow for the neurovascular structure to be identified, it would be marked as Not Applicable.

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Fig. 1 Pin trajectory for robotic total hip arthroplasty array.

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Fig. 2 MRI demonstrating anatomical structures of interest with each given pin zone LFCN, lateral femoral cutaneous nerve; IIA, Internal Iliac Artery; SGN, superior gluteal nerve; SGA, superior gluteal artery.

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Fig. 3 MRI demonstrating the pin trajectory at caudal angles of 35, 45, and 55°.

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The pin entry points were based on the Stryker Mako Total hip array placement using the direct anterior approach technique (Fig. 4). The first pin's entry point was placed 2 cm proximal to the level of the anterior superior iliac spine. The third pin's entry point was 29 mm away from the first pin; the second pin's trajectory was between the first and third pin. The distances from each of the four zones (A1, A2, B1, B2) to each of the four neurovascular structures (LFCN, IIA, SGA, SGN) were obtained for the three pins and the three caudal angles (35, 45, 55). Of these values, the means of the zone with the shortest distance was used for analysis.

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Fig. 4 Mako total hip direct anterior surgical guide ¹⁵.

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For Aim 1, we compared the means of distances from the zone of the pin closest for each neurovascular structure for each caudal angle. For Aim 2, the means of distances were compared based on gender. For Aim 3, the deepest zone of pin seen in bone was assigned a numeric value, with deeper zones being assigned higher values (A1 = 1, A2 = 2, B1 = 3, B2 = 4). The means of distances of the nine patients were compared for each angle of a given pin. Paired two tailed t-test was used for comparing two samples (male versus female) and Analysis of Variance, ANOVA, was used to compare three samples (comparing 3 pins or comparing the 3 angles) using Microsoft Excel Data Analysis. A p-value of 0.05 was used to determine statistical significance.

3 Results

Our study population consisted of nine subjects, six women and three men. The overall mean age was 64.8 years. The mean age for women was 64.7, and for men was 65 years. When looking at overall means, the LFCN was closest to the A1 (most superficial) region for all pins and angles and the IIA was closest to the B2 (deepest) region for majority of pins and angles examined, while there was variability in zones closest to SGA and SGN (Tables 1–4). Caudal angle did not appear to have a significant difference for distance to LFCN. For pin 3, a caudal angle of 35° was the furthest distance away from the IIA (p = 0.005), while no significant difference was found for pins 1 and 2. For pin 1, a caudal angle of 55° was the furthest distance away from the SGA and SGN, while no significant difference was found for pins 2 and 3 (Tables 1–4). After comparing distances to neurovascular structures based on pin number and caudal angle, we did not find any statistically significant differences based on gender (Table 5). Pin 1 placed at a caudal angle of 55° was associated with the deepest portion of the pin being in bone, compared to other caudal angle trajectories. For pins 2 and 3, there did not appear to be a caudal angle associated with a deeper zone of the pin being in bone (see Table 6).

4 Discussion

The use of robotic-navigated arthroplasty initially started with the knee and since has expanded to the hip. It has been estimated that the use of robotic TKA has increased over six-fold since 2017, encompassing approximately 12 % of TKA, while use of robotics in THA has doubled to approximately 5 % of THA.⁶ Existing research on robotic arthroplasty has primary been focused on TKA and ranges from comparing implant placement and patient outcomes to conventional technique as well as defining safe zones of intraoperative array pin placement. ^7,8 Lambers et al. studied iliac crest pins placed for RATHA in 43 patients and noted majority experienced pain from the pin site, which improved by 12 weeks after surgery, but the study did not investigate neurovascular injury. ⁹ Literature has shown robotic arthroplasty's utility for implant placement, reduced leg length discrepancy, and lower rates of dislocation ^10,11 as well as its associated complications regarded as uncommon such as site infection, periprosthetic, and bony debris. ^2,12–14

Although the number of RATHA has been sharply increasing, to our knowledge, there has been no literature regarding safe placement of the robotics array pins and therefore no standardization for pin placement intraoperatively. Therefore, as these pins are usually placed “free hand,” this study sought to determine safe zones to determine which entry angle would be associated with the pin being furthest away from important neurovascular structures and with having the deepest portion of the pin being in bone. The findings of our study suggest that pin 1 would be safest placed at a caudal angle of 55° to avoid injury to the SGA and SGN and pin 3 would be safest placed at a caudal angle of 35° to avoid injury to the IIA. We found no differences in distances from pins to neurovascular structures based on gender, regardless of caudal angle. The results of this study also showed that pin 1 should be placed at a caudal angle of 55° to ensure the deepest portion of the pin is in bone.

This study has several limitations. Although the average age of the patients was similar to those receiving arthroplasty procedures, patients may have varying body and bone morphology, and the overall variability in anatomy may impact the distance from the pin to neurovascular structures. Furthermore, the focus of this study was investigating radiographic measurements, as limited to no literature exists on this topic. The study did not investigate clinical outcomes like blood loss, periprosthetic fractures, or infection. From review of the existing literature and authors' experience, RATHA has been regarded as a safe surgical modality. However, as the number of RATHAs performed increases, we encourage the standardization of array pin placement trajectory to avoid damage to key neurovascular structures.

Author contributions

Conceptualization: Jessyka Desrosiers MD, Aneesh V. Samineni MD, David S. Constantinescu MD, Victor H. Hernandez MD, Jean Jose MD, Data Curation: Aneesh V. Samineni MD, Sarai G. Ramirez MD, Juan D. Lopez MD, Jean Jose MD, Formal Analysis: Jessyka Desrosiers MD, Aneesh V. Samineni MD, Jean Jose MD, Investigation: Jessyka Desrosiers MD, Aneesh V. Samineni MD, Sarai G. Ramirez MD, Juan D. Lopez MD, David S. Constantinescu MD, Methodology: Aneesh V. Samineni MD, Jean Jose MD, Supervision: Jessyka Desrosiers MD, Jean Jose MD, Victor H. Hernandez MD, Writing – Original Draft: Jessyka Desrosiers MD, Aneesh V. Samineni MD, Sarai G. Ramirez MD, Juan D. Lopez MD, Writing – Review & Editing: Jessyka Desrosiers MD, Aneesh V. Samineni MD, Sarai G. Ramirez MD, Juan D. Lopez MD, David S. Constantinescu MD, Victor H. Hernandez MD, Jean Jose MD.

Consent

Informed consent was not required for this study, as it was retrospective in nature and involved no direct patient interaction or identifiable information.

Ethical statement

This study did not require IRB approval as it was determined to be exempt due to its retrospective design and use of de-identified data.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.