Robotic Arm-assisted Augment Preparation (RAAP-Technique) During Total Knee Arthroplasty

Abstract

Modular metal augments are commonly used to reconstruct bony defects during complex primary and revision total knee arthroplasties. The slot for placing an augment is usually prepared using an intramedullary jig and conventional instruments, even during robotically assisted total knee arthroplasty. We report a robotic arm–assisted augment-preparation (RAAP) technique to reconstruct uncontained bony defects using modular metal augments. In this technique, a saw-based robotic arm is utilized to precisely resect the base of the bony defect to accommodate the modular metal augment of appropriate size. The method described in this study avoids the need for conventional revision instrumentation to perform this part of the surgery. Surgery can be completed without removing the robotic array pins. Hence, the robotic system can be retained until the end of the surgery, enabling the monitoring of the final alignment robotically, even during and after the polymerization of the cement. A short case series utilizing this method is also presented.

Introduction

Bony defects of various sizes and shapes may be encountered during complex primary total knee arthroplasty (TKA). Reconstruction of these defects is crucial in restoring the joint line, achieving soft-tissue balance and desired alignment, and ensuring the longevity of the prosthetic joint [1,2]. Methods to reconstruct the defects are filling with bone cement, bone cement with cortical screws, bone grafts, modular metal augments, cones, sleeves, and mega prostheses [[3], [4], [5], [6], [7], [8]]. The applicability of each option depends on the surgeon’s preference, the size and shape of the defect, and the availability and cost [3,[9], [10], [11]]. One commonly used method is modular metal augments [[2], [3], [4]].

Robot-assisted techniques and protocols are currently utilized for primary TKA, and new methods are being developed for various complex primary and revision TKAs [12,13]. Robot-assisted techniques have reported greater accuracy of component positioning, soft-tissue protection, increased patient satisfaction, and a short learning curve [14,15]. The above-mentioned advantages might help improve clinical, surgical, and patient-reported outcomes [14,15]. Even during robotically assisted TKA, the slot for placing the metal augment is prepared using conventional instruments and intramedullary zigs after removing the robotic pins and arrays. Because robotic arrays must be dismantled during this step, the robotic system cannot be used any further for trial reduction, monitoring the alignment during cement polymerization, and acquiring the final alignment values.

This article describes the robotic arm–assisted augment preparation (RAAP) technique for reconstructing an uncontained tibial bone defect (Anderson Orthopedic Research Institute [AORI] Type 2) [9] using modular metal augments. We also report a short case series utilizing this technique.

Surgical technique

Preoperative planning

Patients are assessed clinically. Anteroposterior, lateral, and skyline radiographs of the knee are obtained (Fig. 1). A computerized tomography (CT) scan is also performed using the Mako 2.0 protocol (Mako, Stryker, Fort Lauderdale, FL) [16]. The Mako segmentation process [16] is utilized to plan the initial bony resection and implant sizing (Fig. 2). Bony defects can be assessed and measured using CT scans. This step would help the surgeon to know the approximate size of the augments and to organize the required inventory preoperatively. However, the final defect size can only be confirmed intraoperatively after the initial tibial resection.

Figure 1.

Preoperative radiographs of the knee.

Figure 2.

Preoperative robotic planning.

Surgical preparation and registration

The patient is positioned supine with thigh and foot supports. The knee is exposed using a midline longitudinal incision and a medial parapatellar approach. Femoral pins are placed intra-incisional and intracapsular along the anteromedial aspect of the distal femoral metaphysis (Fig. 3a). These pins are passed at an angle of about 45° from the anteromedial to the posterolateral direction. These pins should not interfere with the saw path required to make the anterior flange and anterior chamfer bony cuts [17].

Figure 3.

Surgical exposure and robotic array placement.

Tibial pins are placed through separate stab incisions distally in the leg (Fig. 3a). These pins must be far enough from the joint line to accommodate tibial stems. For example, if a 100-mm stem is planned to be inserted, array pins must be inserted at least 130 mm from the joint line (Fig. 3b). Otherwise, the array pins will interfere with the preparation and insertion of the stem. This step will allow the robotic system to be retained until the end of the surgery to monitor the final alignment and cementation. The registration process is completed according to the Mako 2.0 protocol. Initial alignment, flexion, and extension gaps were recorded (Fig. 4a and b).

Figure 4.

(a) Deformity in extension. (b) Deformity in flexion. (c and d) Modified intraoperative planning for initial resection.

Intraoperative planning and preliminary bone resection

The preoperative plan is modified intraoperatively after considering the soft tissue laxity and the gap measurements (Fig. 4c and d). The depth of bone resection is adjusted to resect the minimum bone and to achieve equal flexion and extension gaps, using the unaffected lateral tibial plateau as the reference (Fig. 4c and d). The Mako 2.0 protocol is used to balance the knee using intraoperative adjustments to preoperative planning in a standard fashion (Mako, Stryker, Fort Lauderdale, FL). At this stage, the medial tibial bony defect is ignored. Both femoral and tibial bone resections are completed using standard robotic protocols.

Estimating bone defect

The residual bone defect will be clearly visible at this stage. The size of the defect is measured using a ruler (Fig. 5a). The Stryker triathlon system (Stryker, Mahwah, NJ) provides 5-mm or 10-mm tibial modular augments. Based on the depth of the bony defect, a modular augment of the appropriate size is selected.

Figure 5.

(a) Measuring the bony defect. (b) Line a: Akagi line; Line b: lateral extent of the augment.

Planning the bone resection for the metal augment

The medial one-third of the tibial tuberosity is marked on the CT scan during the segmentation process to determine the tibial rotation. Intraoperatively, a robotic probe is used to identify this point on the tibia and confirm the rotation of the tibial component. Akagi line [18] is drawn to determine the anteroposterior axis of the tibial component (Fig. 5b, Line a). Using a trial augment (size as determined in the previous step) as a template, the lateral limit of the proposed bone resection for seating the augment is marked (Fig. 5b, Line b).

Resecting the bone for placing the augment

Depending on the size of the modular augment selected, the robotic technician is asked to add 10 mm or 5 mm of additional resection to the planned initial tibial resection (Fig. 6). For example, if the initial resection thickness is 3.5 mm (Fig. 4c) on the medial tibial plateau, the readjusted resection would be 13.5 mm (Fig. 6), to accommodate a 10-mm augment. The robotic arm is brought into the surgical field again. Checkpoints are verified using the robotic probe. Additional resection is performed using the robotic saw at the base of the defect (Fig. 7a). Robotic bone resection is limited only to the medial tibial plateau (Fig. 6). The lateral extent of the resection is limited to the line marked earlier (Fig. 5b, Line b). Using a saw blade or an osteotome (Fig. 7b), bone is resected vertically down at the lateral extent of the defect (Fig. 5b, Line b) to complete the resection. The resected bone is removed (Fig. 8a). A trial augment is placed in this slot and confirmed to be seated satisfactorily (Fig. 8b). The tibial keel is prepared in a standard fashion.

Figure 6.

Resection of the medial tibial plateau with an additional 10 mm.

Figure 7.

(a) Robotic resection of the base of the defect. (b) Vertical cut at the lateral extent of the defect.

Figure 8.

(a) After the completion of all bony resection. (b) Trial component and tibial stem preparation.

Resecting the lateral hemi-tibia for placement of an augment could be difficult using the Mako saw. To facilitate this step, the patella is retracted laterally using a Hohmann’s retractor, and the patellar tendon is protected at all times. However, the lateral defects are less commonly encountered, and in such cases, a lateral parapatellar approach could be useful, especially in a valgus knee [1,2].

Tibial stem preparation

The trial tibial component and the trial augment are fixed to the proximal tibia using a couple of standard pins. The tibia is prepared for stem insertion using standard intramedullary reamers (Fig. 8b). A trial reduction is performed using all the components and the stem. The knee alignment and the soft-tissue balance are assessed using the Mako 2.0 protocol. Final extension and flexion gaps are assessed and confirmed to be satisfactory. The robotic array pins are retained, and the robotic system can be utilized to monitor the knee alignment until the end of the surgery.

Alternatively, if the array pins are not placed distally beyond the proposed stem length, the robotic array pins must be removed before the stem preparation. A trial reduction is performed using trial components without attaching a trial stem and is confirmed satisfactory using the robotic system. Robotic pins are removed. The tibia is then prepared using intramedullary reamers. In this scenario, the robotic system cannot be used any further for monitoring the final alignment.

Implantation and wound closure

The final components are cemented in a standard fashion (Fig. 9a), and final alignment values are recorded using the robotic system (Fig. 9b). The wound is closed in layers. A postoperative radiograph is obtained (Fig. 10).

Figure 9.

(a) After implantation. (b) Final alignment values.

Figure 10.

Postoperative radiographs.

Brief case series

For this case series, patient consent and institutional review board approvals were obtained. Between September 2022 and December 2024, the author performed about 1500 robot-assisted primary TKAs using the Mako robotic system. Extrapolating the experience of using several reconstructive techniques in complex TKAs, the RAAP technique was developed and utilized in nine TKAs in seven patients during this period. Demographic and surgical details are presented in Table 1. The average age at the time of surgery was 71.66 years (median 72 years, range 61 to 82 years). Two patients were rheumatoid, and all others were suffering from osteoarthritis. All these patients underwent standard preoperative clinical and radiological assessment, including a CT scan as per the Mako protocol. The preoperative deformity was varus in eight patients and valgus in one patient. The most typical defect was AORI [9] Type T2A (Table 1).

Table 1.

Patient demographics and surgical details.

Case	Age (years)	Side	Sex	Diagnosis	Deformity	Preoperative HKA	AORI type	Augment size and type
1	61	Left	F	Rheumatoid arthritis	Varus	24	T2A	10 mm, medial
2	72	Left	F	Osteoarthritis (previous high-tibial osteotomy)	Valgus	7	T2B	10 mm, lateral
3	72	Right	F	Osteoarthritis	Varus	35	T2AF1	10 mm, medial
4	73	Left	F	Osteoarthritis	Varus	21	T2A	10 mm, medial
5	73	Right	F	Osteoarthritis	Varus	20	T2A	10 mm, medial
6	68	Right	F	Osteoarthritis with extra-articular deformity	Varus	24	T2AF1	10 mm, medial
7	66	Left	F	Rheumatoid arthritis	Varus	40	T2A	10 mm, medial
8	78	Left	M	Osteoarthritis	Varus	23	T2A	10 mm, medial
9	82	Right	F	Osteoarthritis	Varus	16	T2A	10 mm, medial

AORI, Anderson Orthopedic Research Institute; HKA, hip knee ankle angle.

All these patients underwent a primary robotic arm–assisted primary TKA using the Mako protocol and Triathlon knee system (Stryker Orthopedics, Mahwah, NJ). Primary bony resections of the femur and tibia are performed in a standard fashion. Then, the size of the residual bony defect is measured. As described in the surgical technique, the robotic protocol is adjusted, and additional bone resection is performed to accommodate modular augments of measured size.

In this case series, all the patients needed a 10-mm modular metal augments and a tibial stem of length 100 mm. Except for one, all patients had medial bone defects. All augments were cemented to the tibial surface, whereas the augment is anchored to the tibial base plate using its locking mechanism. A universal tibial baseplate that can accommodate a stem and modular metal augments (Triathlon, Revision knee system, Stryker Orthopedics, Mahwah, NJ) was used in all cases.

One patient (Case 6) had an extra-deformity of the distal femur due to previous surgery for osteomyelitis during her childhood (Fig. 11). The extraarticular deformity was managed using a single-stage, robotic arm–assisted, intra-articular correction. This patient also required a femoral stem (Fig. 11). Another patient had a valgus deformity following a failed, overcorrected, lateral closing-wedge high-tibial osteotomy performed about 20 years ago (Fig. 12). This patient alone required a lateral augment, an offset bolt, along with a tibial stem (Fig. 12).

Figure 11.

Preoperative (a) and postoperative (b) radiographs of a patient with mild extra-articular deformity in the femur.

Figure 12.

Preoperative (a) and postoperative (b) radiographs of a patient with previous high-tibial osteotomy.

After the surgery, patients were followed up at 2 weeks, 6 weeks, 3 months, 12 months, and every year thereafter. Anteroposterior, lateral, and skyline radiographic views of the knee were obtained during each follow-up to assess for any loosening or migration of implants. Patients were clinically evaluated and monitored for any infection or any subsequent revision. These patients were assessed clinically and radiologically at a mean follow-up of 16.2 months (7 months to 29 months). All the patients were available for the current study. All the 100% modular augments remained in a satisfactory position. There were no mechanical failures related to the implant, augment, or stems. There were no specific complications associated with the current technique. No subjects have undergone revisions for any reason in the operated knees.

Discussion

This study describes a RAAP technique for reconstructing uncontained tibial bony defects during primary TKA. Modular metal augments are used successfully to reconstruct the bony defects during primary and revision TKA using conventional jigs [[2], [3], [4]]. Robot-assisted techniques are now widely used, but only for primary TKA. Even during robotic arm–assisted knee arthroplasties, preparation for modular augments is performed using manual jigs, which are similar to conventional techniques. Our technique can be utilized during a robotic arm–assisted TKA to reconstruct uncontained bony defects using modular augments without requiring manual jigs.

Robotic arm–assisted techniques have shown increased precision of component position compared with conventional techniques with variable degrees of improvement in functional and short-term outcomes [14,15]. Robotic arms are utilized only for the primary bone cuts. Protocols to prepare the slots for modular metal augments are currently not available. Even during robotic arm–assisted TKA, slots for the metal augments are prepared using conventional zigs. That involves using an intramedullary guide, which are usually inserted after removing the tibial pins attached to the robotic arrays. Removal of tibial pins and robotic arrays causes the robotic system to cease functioning. Hence, the trial reduction must be completed before the augment and stem preparation. Because the tibial arrays have already been removed, the assessment of alignment and soft-tissue balance cannot be done using the robotic system in the lateral stages of the surgery. As the arrays are removed, the surgeon cannot utilize the robotic system to monitor the knee alignment during and after the polymerization of the bone cement.

Even though it could be possible to use an intramedullary guide to prepare the stem, if the array-pins are inserted in the distal third of the tibia, our method avoids the use of conventional instrumentation for this step of the surgery. Switching to the conventional method requires additional instruments, increasing the operation room inventory. The technique described in this study will not require any additional instruments; it is quicker and may be more precise because of the use of a robotic arm. This method may also be utilized for defects in the distal femur using similar calculations and techniques.

Bone defects around the knee are commonly classified using the AORI classification system [9]. This system is based primarily on the size of the bone defect in the tibia and femur. Modular metal augments are commonly used to reconstruct bone loss [1] for AORI Type 2 defects. Even though both wedge and rectangular-shaped augments are available, biomechanical studies have indicated that rectangular blocks are superior to wedges since they could directly transmit torsional load to the bone, reducing cement mantle strains between the baseplate and the tibial plateau [10,11]. In the technique described in our study, tibial resection can be performed at the base of the bony defect to seat a rectangular metal block of the desired size. In addition, a stem is used in all the patients to provide additional stability, improve the distribution of shear stress, and reduce stress at the bone-cement interface [2,19,20].

Augment preparation can also be performed using other robotic platforms with robotic revision protocols (CORI, Smith and Nephew, Memphis, TN), even though they are not widely available. Cochrane et al [21] reported using an imageless, bur-based system for revision TKA. In this technique, pre-templated bone cuts are made initially using the robotic system, and the need for augmentation is assessed based on any identified bony defect. Bony cuts are completed, and augments are prepared using a robotic burr [21].

We believe the RAAP technique will add to the robotic protocols used in complex knee reconstruction. Even though we need long-term, randomized, prospective studies to prove the benefits of the current technique, if we extrapolate the currently available data for utilizing the augments in bony defects around the knee, the current technique should only facilitate their use.

Conclusions

This study presents a new technique for preparing the uncontained tibial plateau defect for seating a modular metal augment during a robotic primary TKA. When an augment is indicated, the RAAP technique can be utilized in AORI type 2 defects. Further evaluation of this technique in more patients, multicenter studies, and a prospective comparison with other methods are needed to confirm its usefulness. They may pave the way for further advances in robotic techniques, and the current technique may be utilized in revision TKA to reconstruct the bony defects when revision TKA applications are developed in the future.

Conflicts of interest

Dr Narayan Hulse has a consulting role with Stryker for educational and training activities.

For full disclosure statements refer to https://doi.org/10.1016/j.artd.2025.101725.

CRediT authorship contribution statement

Narayan Hulse: Writing – review & editing, Writing – original draft, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.