Technique: Uni-knee resurfacing with patient-specific implants and instruments

Advances in imaging allows for a bone-preserving resurfacing solution using disposable instruments.

The iUni implants (ConforMIS) and instrumentation are provided in a single sterile tray.

Unicompartmental knee arthroplasty (UKA) has enjoyed a surge in popularity recently due to better long-term results reported in young, heavier patients, better implant designs and improved surgical techniques. In fact, Riddle et al reported that the incidence of UKA is growing at triple the rate of total knee replacement in the United States.

This article describes a surgical technique for implanting a novel partial knee resurfacing UKA using customized, single-use instrumentation.

The iUni (ConforMIS) utilizes CT scans of a patient’s knee and partial scans of the hip and ankle to create patient-specific implants and instrumentation. The implants are designed using proprietary technology that digitally recreates the knee anatomy, maps the surface topography of the femur and tibia, and corrects for axis deformity. The same data is used to create cutting and placement guides that are pre-sized and pre-navigated to work with the patient’s anatomy and custom implants.

The kit is provided by the manufacturer with the resurfacing implants and disposable instrumentation in a single sterile tray (Figure 1).

Patient positioning, prep

The patient is positioned supine on the table with the leg resting on a foot support around 90° flexion. After a standard, short midline skin incision, a medial or lateral parapatellar arthrotomy is performed. The medial (or lateral) sleeve is not released, but all femoral and tibial osteophytes, including those in the intercondylar notch are removed.

In extension the sulcus terminalis is marked with a marking pen, where the leading anterior tibial surface makes contact with the femur. The femoral cutting block, which is shaped to fit the condyle and represents the size and geometry of the femoral implant, is placed on the femoral condyle. In most cases the anterior edge of the femoral cutting block seats about 2 to 3 mm inferior to the sulcus terminalis. The round anterior silhouette of the femoral cutting block is marked on the femoral condyle.

The implant is designed to the surface of the subchondral bone with a thickness of 3.5 mm. Since it resurfaces the bone plate, all cartilage posterior to the sulcus terminalis, including the posterior condyle, must be removed. This is facilitated using a 10-mm blade, curved elevator, osteotome or a ring curette (Figures 2 and 3). Confirm that the femoral jig conforms to the condyle after cartilage removal.

Once removal of all residual cartilage on the femur has been concluded, scrape all residual cartilage off the tibial plateau and start balancing the knee.

Cartilage removal on condyle is facilitated using a 10-mm blade, curved elevator, osteotome or a ring curette. Images: Fitz W

Balancing of the knee

There are four navigation “chips” of varying thicknesses in 1-mm increasing increments included in the instrument tray. Each chip has an underside that matches the exact shape and topography of the patient’s tibial surface. When inserted into the compartment, the chip will self-seat into a stable position due to its conformity with the anatomic landmarks on the tibia. The top surface of this chip is flat to allow referencing off the distal femoral condylar surface during balancing (Figure 4).

Each chip is inserted, from thinnest to thickest, with the knee in flexion and then taken through ROM. Select the chip that provides optimal ligament tensioning (Figure 5).

An opening under valgus stress of about 1 mm is recommended medially in extension and 90° flexion and about 1 to 3 mm under varus stress laterally in extension and about 3 to 5 mm in 90° flexion. The thicker the chip, the less bone is resected off the tibia.

The top surface of the navigation chip is flat to allow referencing off the distal femoral condylar surface during balancing.

Each navigation chip is inserted, from thinnest to thickest, with the knee in flexion and then taken through ROM.

The tibial jig is attached to the navigation chip to establish its placement.

The axial tibial cut is performed referencing off the tibial jig.

The femoral jig is designed to conform to the femur in only one location to aid in proper positioning.

The femoral cutting block is pinned in place and the posterior femoral condyle is resected with an oscillating saw.

Axial and sagittal tibial cuts

The selected navigation chip connects to a tibial cutting block. The tibial jig connects to an extramedullary alignment guide by sliding the dovetail feature of the tibial jig onto the top of the alignment guide. Place the alignment guide on the leg (Figure 6) and attach the tibial jig to the navigation chip to establish its placement.

Verify and confirm the position of the tibial cutting block since all three planes, the horizontal cut (90° relative to the tibial mechanical axis), the sagittal cut and the posterior slope are determined in this step. Lock all positions on the extramedullary guide. You may use Kocher forceps to lock the pin in position.

Perform the sagittal tibial cut using the tibial jig. The reciprocating saw blade can be left in to protect the ACL insertion while performing the axial cut. Perform the axial tibial cut referencing off the tibial jig (Figure 7). Remove the alignment guide and the tibial jig.

Femoral preparation

Place the femoral jig on the distal femur and verify its position. The femoral jig is designed to conform to the femur in only one location to aid in proper positioning (Figure 8). Remove any additional cartilage or osteophytes that were missed in step 1 until the fit is snug and secure.

There are two things to consider: the peg-holes are drilled in 15° flexion relative to the sagittal anatomical femoral axis; and there is only one bone resection required on the femur — a 3- to 4-mm resection of the posterior condyle (including the thickness of the saw blade). Drill and pin the femoral cutting block in place. Complete the posterior femoral condyle resection with an oscillating saw (Figure 9).

To complete femoral preparation, an anterior recess is prepared using a curved osteotome or a 5-mm burr. The most anterior edge of the component submerges 3.5 mm below the subchondral bone plate. The taper starts 10 mm inferior to it. Also, the transition from the subchondral bone to the anterior edge of the posterior cut has to be rounded using a file, burr or osteotome. Verify smoothening of the edge and placement and depth of recess with the femoral component.

With the femoral trial in place, the 8-mm spacer block is inserted and balance in flexion and extension are evaluated.

The tibial template is placed on the tibia and both holes are drilled, pining the anterior hole only to accommodate instruments for the upcoming fin hole preparation.

Multiple 1.5- to 2-mm cement holes are drilled to enhance cement interdigitation with femoral cortical surface.

Balancing and tibial prep

A femoral trial component and a balancing block are utilized to assess balancing. With the femoral trial in place, insert the 8-mm spacer block and evaluate balance in flexion and extension (Figure 10). A joint play of 1 to 2 mm is recommended for the medial iUni in extension and flexion. For lateral iUnis, we recommend 1 to 3 mm in extension and 3 to 5 mm in 90° flexion. If the knee is too tight, resect an additional 1 to 2 mm from the tibia. If too loose, insert the 10-mm spacer block and evaluate balance in flexion and extension.

Place the tibial template on the tibia and drill both holes, pining the anterior hole only to accommodate instruments for the upcoming fin hole preparation (Figure 11). Next, create the fin hole using a 5-mm osteotome. The tibial implant is designed to match the patient anatomy exactly and should cover the entire tibia cortex without overhang or undercoverage. The outline of the tibial template provides visual confirmation of the match.

Trialing, cementing of implant

Drill multiple 1.5- to 2-mm cement holes to enhance cement interdigitation with femoral cortical surface (Figure 12) and thoroughly irrigate the joint. Next, place the metal implants into position and insert the trial poly which provides optimal balancing. Two different thicknesses are provided, 6 or 8 mm. Combined with the 2-mm thickness of the tibial tray, the 6- and 8-mm trials will correspond to the 8- and 10-mm spacer blocks used to confirm proper balance.

Cement the tibial tray first, remove all extruded cement and insert femoral component. Bring knee in 45° and insert trial tibial insert allowing equal pressurization of the femoral component in flexion and in extension. Cement the femoral component, remove all extruded cement and await complete cement hardening (Figure 13). Remove trial insert and any residual extruded cement and insert the actual polyethylene insert (Figure 14). Wound closure of the arthrotomy is recommended in flexion and in multiple layers.

The femoral component is cemented and all extruded cement is removed.

Once the trial insert and any residual extruded cement are removed, the actual polyethylene insert is inserted.

For more information:

• Wolfgang Fitz, MD, can be reached at Brigham and Women’s Hospital, 850 Boylston Street, Suite 130, Chestnut Hill, MA, 02467; 617-732-5322; e-mail: wfitz@partners.org. He is a scientific advisor and a developer of the implant and receives royalties and stock options from ConforMIS and research or institutional support from DePuy.
• Tom Minas, MD MS, can also be reached at Brigham and Women’s Hospital; e-mail: tminas@partners.org. He is a scientific advisor and a developer of the implant. He receives royalties and stock options from ConforMIS and institutional or research support, miscellaneous funding and is a consultant or employee of Genzyme.

References:

• Pennington DW, Swienckowski JJ, Lutes WB, Drake GN. Unicompartmental knee arthroplasty in patients sixty years of age or younger. J Bone Joint Surg Am. 2003;85:1968.
• Riddle DL, Jiranek WA, McGlynn FJ. Yearly incidence of unicompartmental knee arthroplasty in the United States.J. Arthroplasty. 2008:23;(3):408-412.