Surgical technique of metaphyseal sleeves and cones
In Part 1 of this newsletter we outlined how metaphyseal fixation of revision TKA can be performed with structural allograft, cones, or sleeves. Nowadays, most surgeons prefer cones or sleeves over allograft, so we have focused on these two treatment techniques here. In addition to describing the surgical techniques, we also interviewed one advocate for cone and one for sleeve technique with the aim of providing a balanced overview of each technique.
Surgical technique using cones
After removing the previous components, the remaining bone is thoroughly cleaned of residual cement, fibrous, and nonviable tissue. This facilitates a correct assessment of the actual bone loss to properly prepare the bone for the correct cone size. While residual tissue may hinder osseointegration of the ingrowth surface, residual cement may even damage or deflect the instruments used to prepare for the cone during bone preparation creating excess bone loss or even fracture.
The position of the bony defect in relation to the intramedullary canal can be estimated with trial cones. The trial cones can be placed upside down on the bone surface over the reamer to assess the size and orientation of the defect and correct placement of the cone.
Various manufacturers provide different methods to guide alignment. All systems have in common that they use the medullary canal as the reference point to fix the guide. Depending on the manufacturer, fixed templates or variable dials are attached to the initial guide (Figure 1), which then allow exact placement of the instrumentation.
Figure 1. In this cone system a cannulated tibial cone reamer, calibrated by cone size, is inserted over an intramedullary reamer which is used for medullary canal referencing.
Redrawn from Surgical Protocol of the Triathlon Revision Knee System, Stryker Corporation, Mahwah, New Jersey.
Once the guide and the template are in place, the cavity is prepared by cutting at a trajectory that mills or reams the bone to exactly match the shape of the implant. Since cones achieve their stability through press-fit, a good seating of the cone in the bone (Figure 2) is paramount for primary stability, which is the basis for osseointegration (Figure 3).
Figure 2. Central, porous titanium metaphyseal tibial cone, with intimate and circumferential host bone contact.
Image courtesy of Bryan Springer.
Figure 3. Tibial metaphyseal cone osseointegration demonstrated by a “spot weld” (arrow) 5 years after revision TKA using a tibial metaphyseal cone and short-cemented stem construct.
X-ray courtesy of Bryan Springer.
This is done with sequentially sized reamers, broaches, rasps, or burrs in a stepwise manner. It is important to pay particular attention to the amount of bone that is being removed. Enough bone should be removed to create a circumferential fit of the cone, however, one must ensure that bone is not removed excessively, because this may increase the risk of fracture.
While there is considerable variation in the techniques between the individual manufacturers, the instruments possess marks, which delineate the size and depth of the corresponding cone.
There may be cases in which the shape of the defect makes it impossible to create a perfect circumferential press fit on all sides of the cone due to bone loss being more prominent on one side compared to the other. In such situations, voids may be filled with morselized graft, demineralized bone matrix, or a suitable synthetic bone substitute. This is tightly packed around the cone until maximal stability is ascertained. This maneuver is not only to enhance the primary stability but also to prevent cement from flowing into the interface between cone and host bone when the final component is cemented into position.
Next, after thorough pulse-lavage to clear all residual debris from the bony surfaces, the cone is positioned on the bone and alignment is double-checked. It is particularly important to ensure proper rotation of the cone in the tibia. Many systems allow for rotation of the tibial component within the cone to ensure proper rotation. However, setting the cone outside of these boundaries will require the tibial component to be malrotated. Once the correct rotation is confirmed, it can be impacted in place. Because the instrumentation allows for a more precise preparation of the bone, the real cone implant should fit securely into the prepared bony bed. Upon impacting, it is important not to use too much force to seat the implant, especially in sclerotic bone, because this could lead to fracture. In general, the size of the real cone implant may be slightly larger than the trial cone due to the porous surface so steady impaction should be utilized to avoid fracture.
Once the cone is firmly seated in the bone (Figure 2), the remaining prosthetic components can be cemented into place. The individual steps of assembly vary between the different manufacturers. In any case, it is most important to carefully verify the rotational alignment of the tibial component during cementation.
Surgical technique using sleeves
As for cones, the first step after removal of the previous components is to thoroughly clean the remaining bone from residual cement, residual granuloma, as well as from fibrous and nonviable tissue. In case the bone is sclerotic, it needs to be loosened with a burr to reduce the risk of malalignment or fracture.
The next step is to open the medullary canal with a straight hand reamer. There are marks on the reamer indicating the reaming depth, which should correspond to the length of the prosthetic stem extensions determined preoperatively. After the initial opening of the canal to the desired depth, reaming is continued with a tapered reamer to which a provisional stem is attached. This provisional stem will act as a guide in the diaphysis to ensure the axis is correct, comparable to a keel that sets the path. Ideally on the tibial side, the size of the stem diameter should not exceed 12–14 mm. This size limit is recommended because in the case of a future revision surgery, only stem sizes up to 14 mm can be easily removed through the opening in the ingrown sleeve. Due to the larger femoral diameter, the femoral stems can be larger.
Once the bone has been reamed to the desired depth, it is further prepared with a broach. This procedure is very similar to the preparation of a hip stem. Broaching creates a bed for the implant, and compacts rather than removes the cancellous bone, which is beneficial for achieving a secure press fit.
Sleeves enable the surgeon to separate the needs of fixation and bony tibial coverage because the placement and orientation of the sleeve is independent from the orientation of the implant. Upon assembling the final construct, the implant can be rotated on the sleeve up to 20° each way.
Broaching the femur is performed in a similar fashion as broaching the tibia. However, finding the right direction for broaching the femur can be difficult. Depending on the individual femoral bow, simply following the path set by the stem in the medullary canal could result in malalignment or an increase in the flexion gap. Therefore, care should be taken to closely follow the posterior cortex by dropping one's hand while broaching, since this will help to align the sleeve correctly and prevent increasing the flexion gap.
Using the previously selected provisional stem as an intramedullary guide, the broach is impacted into the bone until the top surface (tibia) or the etch mark (femur) of the broach is at the desired resection level. The broach should be impacted firmly, but not too aggressively to avoid fracture.
This procedure is repeated with consecutively larger broach sizes until cortical chatter can be felt upon impaction, which indicates a firm fit. Ideally, one should have a light scratch fit rather than a very tight fit. The fit should also be tested manually. In the tibia, it should be possible to lift the tibia off the bed just with the broach handle. In the femur, torque to the broach should move the leg without creating any motion at the broach/bone interface. These tests ensure adequate provisional fixation with the broaches and the sleeve.
Next, the handle can be removed, and further bone cuts as needed for final implantation of the prosthesis can be made. Should there be any visible voids between broach and bone in spite of the firm fit, they should be filled with morselized bone graft to improve implant apposition to bone and thus facilitate biological fixation. Once the broaching has been completed and the trial broaches are firmly in place, they can be used as an aid for final preparation. In case the joint line needs to be adjusted, larger sized femoral or tibial sleeves can be used for distalization or proximalization.
For final implant insertion, the same care should be taken for impaction as with the broaches. Final implantation can be performed with sleeves alone or preferably, the sleeves can be impacted onto the femoral and tibial implant on the back table to ensure adequate seating of the sleeve onto the implant. And then the combined implant sleeve and stem can be impacted at one time. The pros and cons of stems versus no stems have been discussed in detail in Part 1 of this newsletter. Cement can be used to fill any defect between the undersurface of the tibia and the femur, but care should be taken to not allow cement to interfere with the biological fixation afforded by the metaphyseal sleeves.
Figure 4. Checking the correct size of the broach (a). Firm but not too aggressive impaction of the broach (b).
Figure 5. Tibia prepared for implant insertion (a). Femur prepared for implant insertion (b).
Figure 6. AP (a) and lateral (b) postoperative x-rays. Note that stems do not function as diaphyseal filling, rather as a keel for added alignment and stabilization to encourage ingrowth.
We interviewed David Dalury and Bryan Springer about their preferences for and use of sleeves and cones.
David Dalury prefers sleeves over cones. Why?
I primarily prefer sleeves over cones due to the instrumentation. It is similar to implanting a femoral stem for a total hip replacement; the sleeves have a 30-year track record of use and their stepped design has been shown to convert impaction forces from shear into compression forces, as has been shown for the S-ROM femoral stem.
Additionally, in case removal should be needed, only having to disrupt the bone-implant interface rather than the cone-bone and cement-cone interface is preferable.
Bryan Springer prefers cones over sleeves. Why?
It is important to remember that both cones and sleeves work well and have excellent clinical results. The advent of metaphyseal fixation, whether through cones or sleeves, has addressed a major issue in revision TKA fixation, and the results have shown reduced aseptic loosening.
My main preferences for using cones over sleeves:
- They are independent on the implant where sleeves are unitized to the implant. This allows me to address the defect separately from the components that I am using.
- Cones can address a larger variety of defects than sleeves in my opinion. They come in different shapes and sizes and can be used for a smaller central defect and larger peripheral contained and uncontained defect.
- The surgical technique for cones has become very streamlined.
- Because they are not unitized, cones do not dictate the position of the implants that oftentimes sleeves can.
- I can use cemented or cementless stems, sleeves generally require cementless stems.
- Sleeves currently are only from one system, whereas multiple companies have cones.
David Dalury: You prefer sleeves. Have you tried cones? What did you dislike and what did you like?
I have used cones occasionally. A positive aspect of cones is that they can fill large cavities, after all, there are rare cases where the largest sleeve is not big enough and a cone is needed. There is nothing special I dislike about them. They involve more freehand introduction and I do have concerns about the need for removal someday.
Bryan Springer: You prefer cones. Have you tried sleeves? What did you dislike and what did you like?
I have used sleeves once or twice, when I was leaving in place the femoral component, which was from that particular company that only had sleeves as an option. A positive aspect of sleeves is that the broaching technique is something most surgeons are familiar with. I could not say I dislike anything in particular about sleeves, but for my workflow and the reasons laid out above, I prefer to use cones.
David Dalury: What do you like most about implanting sleeves?
I like the familiarity of the procedure: the reaming is identical to femoral stem implantation for hip replacement and it is very quick and predictable.
Bryan Springer: What do you like most about implanting cones?
First, I like the ease of use and the ability to easily address all defects. Second, I like the reliability of fixation.
David Dalury: What is the most difficult part when implanting sleeves and how does this compare to cones?
The hardest part is making sure that when there is sclerotic bone on the tibia that you remove this prior to impaction to avoid the risk of fracture…it has not happened to me yet! Compared to cones, in my view, there are still better instruments with sleeves.
Bryan Springer: What is the most difficult part when implanting cones and how does this compare to sleeves?
In many cases, the bone can become thin and sclerotic. You have to be careful not to overream. These cones should rest against cancellous bone when possible but oftentimes it requires reaming out to cortical bone. This places the patient at increased risk for fracture when prepping and impacting the cone.
David Dalury: Is it difficult to revise sleeves?
Not easy but you can disrupt the Morse taper and remove the tray and stem (avoid using stems bigger than 14 mm so they can be removed) and there is a tool that works well on breaking up the bone-implant interface.
Bryan Springer: Is it difficult to revise cones?
Yes. If the cone is well fixed it can be extremely difficult to remove.
David Dalury: What needs special attention in case you need to revise sleeves?
You need to take your time, but it can be done.
Bryan Springer: What needs special attention in case you need to revise cones?
First, a pearl when you are inserting them: In general, I try to sink the cone just below the tibial surface. This allows an interface to be able to work in between the cone and the tibial tray that will aid in removal.
Removal of the cone can be difficult and often requires extensive exposures. I start with the same principles as trying to just remove the tray: circumferential exposure of the tray at the cement-implant interface. They try and axially extract the tray. If this is unsuccessful, I will often then do a tibial tubercle osteotomy to allow exposure to the anterior aspect of the cone to free it up. On occasion, you will have to cut the tray from the top to expose the cone.
David Dalury: Have you ever had to revise a colleague's cones? If so, what was the most challenging part?
Hugging the contour of the cone so as to remove it without damaging the cortex of the tibia. The tibia is more difficult than the femur (can do an osteotomy on the femur for removal if needed).
Bryan Springer: Have you ever had to revise a colleague's sleeves? If so, what was the most challenging part?
Same principles as above for cones.
David Dalury: What tips would you give less experienced surgeons to help them correctly implant sleeves?
Avoid the larger stem sizes (see above regarding need for removal), stop increasing size of sleeve preparation once you can lift the leg off the bed, or torque the tibia with the broach in place; make sure any remaining sclerotic tibial bone does not limit ability to seat the broach (use a burr or rongeur to remove that first). Overall, they are very easy to use.
Bryan Springer: What tips would you give less experienced surgeons to help them correctly implant cones?
The biggest mistake I see people make is overreaming, making the defect bigger than it needs to be and running the risk of fracture.
This series of articles was created with the support of the following specialists (in alphabetical order):
This article was compiled by Elke Rometsch, Project Manager Medical Writing, AO Foundation, Switzerland.
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