The Uncemented Evolution.
A new method for total elbow arthroplasty.
Design Philosophy
The overriding design goal was to reproduce the normal anatomy and biomechanics of the human elbow while not using cement to gain component fixation. We derive secure connectivity between implant and bone through intramedullary screw fixation. We created an implant that closely matches the typical dimensions and architecture of the distal humerus and proximal ulna. Secure polyethylene fixation is achieved with a mechanism that allows subsequent replacement of this wear surface. Three different implant sizes were designed to match the patient's anatomy. Reproducing anatomy extends to maintaining the radial head as well as the medial and lateral collateral ligaments, which are the primary soft tissue stabilizers of the elbow and are reconstructed during the implantation of this total elbow arthroplasty.
Design Features
The Kaufmann Total Elbow (KTE) arthroplasty is not cemented and uses intramedullary screws to gain purchase into the threaded intramedullary canals of the humerus and ulna. One cross locking screw in the humerus and two screws in ulna augment the construct’s stability.
In distinction to semiconstrained implants, this design does not exhibit a direct mechanical pin linkage between the humeral and ulnar components. The snap fit hinge employs laxity that allows for varus and valgus deviation while also providing proprioceptive feedback to the surgeon when implanting the components as it was designed to mimic the capsule that surrounds the elbow.
Elbow stability is derived from the ligament reconstruction, which is designed to transmit forces between the humerus and ulna and aims to mimic the primary static stabilizers of the elbow. The medial and lateral collateral ligaments are considered essential stabilizers and are reconstructed during implantation of this total elbow arthroplasty.
This uncemented elbow replacement requires osseointegration for the bone to secure the implant. Because this arthroplasty design is stabilized with a ligament reconstruction, ligament incorporation into bone is also required. While both of these processes happen, elbow replacements will face biomechanical challenges and this healing environment needs to be protected with a hinged brace and activity modification. We believe that substantial ligament healing will occur in three months and that bone incorporation will occur before that.Preventing Stress Shielding
Stress shielding occurs when forces that are usually experienced by the elbow are not experienced to their full extent; a process that leads to the bone and soft tissues becoming weaker over time. Wolf’s law and Davis’s law govern these behaviors.
Stress shielding due to nonanatomic force transmission may lead to failure in semi constrained replacements.
When bones are stress shielded the mineral density decreases over time.
During everyday use, the collateral ligaments of the elbow are tasked with transmitting varus and valgus forces that occur when the arm is moved away from the patient’s side.
Non-absorbable suture augmentation of ligament reconstructions has seen an increase in use. Although beneficial for initial healing, stress-shielding of the ligament via non-absorbable suture augmentation may ultimately impact the final strength and composition of the reconstructed ligament.
KTE Design aims to avoid stress shielding.
The KTE arthroplasty design aims to prevent stress shielding through the re-creation of force transmission in a manner that is typical for the elbow. This is accomplished by:
1. Designing an elbow arthroplasty that is not mechanically linked beyond the snap fit connection that aims to re-create the integrity of the capsule.
2. Mandating a reconstruction of the ligaments that stabilize the elbow. This effort attempts to mimic the biomechanical situation that exists in the native elbow whereby the ligaments are tasked with transmitting physiological forces. Once the ligaments transmit as much force as possible, so too will the respective origin and insertion sites of those ligaments experience force, which has the goal of minimizing stress shielding of the distal humerus and proximal ulna.
3. Using absorbable suture when augmentation of the ligament reconstruction is being performed, which encourages that, once the sutures have absorbed, the ligaments experience the full force that is transmitted through them. The KTE ligament reconstruction only uses absorbable PDS suture.Humeral Body Component Implantation
The Intramedullary canal is identified first. A rongeur and a pilot drill bit are used to create a hole within the trochlea that roughly corresponds to the course of the IM canal of the humeral shaft.
Five different diameter drill bits will accommodate the variability in the inner diameter of the intramedullary canal. Custom drill bits were created that have a smooth leading (cutting) edge that aims to not engage the inner cortex. The drill head was designed to "find its way" within the intramedullary canal. The humeral implant uses drill bit diameters of ¼”, 5/16”, 0.368”, 27/64”, and 17/32”, which will correspond to IM screws that have 5/16-18, 3/8-16, 7/16-14, 1/2-13 and 5/8-11 threads. The smallest diameter drill bit is used first and then increasing diameter drill bits are utilized as needed. The drill needs to first find the intramedullary canal and then remove primarily cancellous bone. It is imperative that during drilling of the IM canal, the drill is not engaging too much bone. Once resistance is met, the drilling effort is halted.
When drilling the humerus a ¼” drill bit is used first. Finger placement on the dorsal and volar shaft provides proprioceptive feedback regarding the trajectory of the drill bit. If chatter is felt with this drill bit then it is left in place and no additional drilling is needed.
If no chatter is appreciated with the smallest bit (¼”) then larger drill bits (5/16”, 0.368”, 27/64”, 17/32”) are used until chatter is felt. Once the last drill bit has been employed that reaches the inner wall of the intramedullary canal the larger diameter drill bit is replaced with the 5/16” diameter drill bit, which now acts as a guide. The 5/16” diameter drill bit is advanced roughly 4” within the IM canal of the humeral shaft and acts as a determinant of the location for the long axis of the humeral shaft.
The centerline of rotation is addressed in the next step. The center of the capitellum is used on the lateral side. The anterior inferior aspect of the medial epicondyle is used as a reference on the medial side. K-wires are drilled into bone ensuring that they do not interfere with the intramedullary drill bit.
At this stage, the centerline of the intramedullary canal of the humeral shaft as well as the centerline of ulnohumeral rotation has been identified. Successful implantation relies on accurate characterization of these anatomical landmarks as the subsequent steps use these features as guides for cutting efforts.
The distal humerus should have a 5/16” diameter drill bit that is advanced within the intramedullary canal as well as two K-wires along the centerline of ulnohumeral rotation. It is important that K - wire tips are not too close to the IM drill bit to avoid damage to the K - wires during subesequent bone cutting.
Three custom cutting guides were designed to match the dimensions of the three sizes (S, M, L) of the distal humerus body components. Each cutting guide incorporates a hole for the IM drill bit that is located within the humeral shaft. On each sides are holes that accommodate 0.45 K - wires that are placed by the implanting surgeon and used to aid in the parallelism of this cutting guide with the K - wires within the distal humerus along the centerline of ulnohumeral rotation.
The next step involves placing the cutting block in line with the intramedullary drill bit. The medial and lateral K - wires that were placed in holes within the cutting block assist with ensuring proper alignment. The cutting block K-wires are then used to rotate the block so that its K - wires are parallel with the ulnohumeral centerline of rotation K-wires that were drilled into bone.
The cutting block is pinned into the distal humerus.
A sagittal saw is used to cut the anterior bone.
A parallel cut that is flush with the cutting block is created with the sagittal saw.
The cutting block is now removed. Parallelism between the ulnohumeral centerline K-wires and the cut surface is the foundation for successful KTE implantation.
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The appropriate size implant is then placed on this surface so as to once again ensure that the morphology of the distal humerus is reproduced by the implant dimensions.
The cutting block dimensions are identical to the outside of the humeral body component. The cutting block is placed so that the grooves in the block line up with the medial and lateral K-wires denoting the centerline of rotation. as well as the long axis of the intramedullary drill bit. It must be verified that, when the cutting block is correctly positioned, medial and lateral bone is present on each side of the cutting guide. If that is not the case then a smaller implant size must be placed.
The medial and lateral extent is marked with a pen on the cut surface.
The guide plate is placed on the anterior flat surface. The plate has a cross in the middle, which is used to visually align the IM axis of the humeral shaft and the K-wires that denote the centerline of ulnohumeral rotation. The distal holes are marked with a pen, which denote the distal extent of the sagittal saw cutting effort.
The sagittal saw is used to cut the bone within the confines of the pen marks. Multiple saw blade passes are needed. This step must proceed carefully and bone intact bone must be present on medial and lateral sides of the implant to ensure ligament to bone healing.
Do not saw more bone medially or laterally than the extent marked by pen on the distal humerus.
After the cuts have been made, the block should fit nicely in this cavity and the intramedullary drill bit should fit comfortably within the cutting block.
The sagittal saw is used to remove the distal bone while again using the block as a guide.
The cutting block dimensions are identical to the distal dimensions of the humeral body component. Once all of the bone cuts have been successfully completed, the broaching effort may commence.
Once the saw cuts have been made the implant can once again be placed into its approximate location to verify proper seating. At this time, the humeral body component should slide freely into the cavity created with the sagittal saw.
Interchangeable linear broaches were designed to remove bone from the medial and lateral sides of the distal humerus.
The medial bone is removed with these custom broaches starting with the smallest size and ending with the broach that corresponds to the desired implant sizes.
The lateral bone is removed by also starting with the smallest size (XS) and advancing to the size that corresponds to the chosen implant size.
A second linear broach was designed to resemble the dimensions of the implant itself.
This linear broach can be used to verify the seating of the implant.
Once the broaching is completed the intramedullary canal is tapped with custom taps that correspond to the intramedullary drill bit that was used.
The marking on the tap can be used to find the length of the intramedullary screw.
Standard tapping methods are employed. Care must be taken to gently begin this process so as to not allow the tap to get stuck and potentially cause a fracture of the humerus.
An intramedullary screw is then used to secure implantation by gaining purchase in the threaded intramedullary canal and pulling the implant into the humerus.
A variety of intramedullary screws are available for use. The screw diameter corresponds to the intramedullary drill bit that gained chatter within the intramedullary canal. The lengths for each diameter are 1.75”, 2.75” and 3.75”. The depth of the tap is used to identify the correct screw length.
The implant is securely seated by tightening the IM screw with a long handle socket wrench.
It is imperative to not overtighten the intramedullary screw as this may lead to a fracture. Two finger tightness is recommended.
The implant is now securely seated .
The holes for the ligament reconstruction are drilled through the medial and lateral epicondyle once the implant has been secured. The process begins with a small pilot drill bit.
Ulnar Body Component Implantation.
After the appropriate soft tissue dissection has been completed, the first step for the insertion of the ulnar component involves the creation of a hole at the base of the coronoid.
A high speed burr is used to remove the bone that is located where the ulnar component will reside. After this hole allows entry into the olecranon, the burr is used to remove subchondral bone within the confines of the medial and lateral cortex within the olecranon and proximal coronoid.
The smallest specialty drill bit is used to find the intramedullary canal within the ulna.
Larger drill bits can be used for the proximal ulna that will receive the body component. As a rule, the drill bit used should not be larger than the body component. Drill only the extent of the body component so as not to compromise the IM screw placement.
Custom designed taps create threads within the IM canal of the ulna. The shaft diameter is designed so that a Synthes adjustable T handle chuck can be used and advanced along the shaft to create handle fixation at variable distances from the working end of the tap. The shaft has measurement notches that allow for visual assessment of how far the tap has progressed.
The ulna is then tapped with a hand held tap that corresponds to the drill bit that was used in the last step. The hand-held tap is used to create threads in the intramedullary (IM) canal.
A rotary ulnar broach was designed that would match the geometry of the polyethylene. It is attached to a drill and removes only bone that is needed to properly seat this component. In this process, the broach is rotated and pressed into the olecranon to cut an axisymmetric shape. The cut depth governed by the application of pressure by the surgeon while the rotary broach is spinning.
Use the rotary broach that corresponds to the implant size. The broach is used to remove bone where the polyethylene wear component will be placed. Remove bone so that the broach centerline of rotation corresponds to the centerline of rotation for the ulnohumeral joint.
The rotary broach is positioned so that the cutting surface is rotated 7 degrees toward the radius. This maintains the 7 degree valgus alignment of the ulnar body component relative to the polyethylene.
A custom hand held broach is used in conjunction with the hand held rasp. The broach is the same size as the implant and can be used to ensure line to line seating of the implant in the proximal ulna.
A rasp is used to remove additional bone needed to place the body component. This step may take some time.
A hand held screw driver gains purchase into the head of the screw and then, with screw rotation, the advancing screw pulls the implant into a sturdy location within the proximal ulna. The implant will seat itself as it is pulled by the advancing IM screw. The ulnar body component exhibits a wedge shape, which will create significant hoop forces during implantation. IM screw tightening advances the implant and must occur without using substantial torque so as to prevent an ulna fracture. Rasping and broaching efforts must ensure that the body seats fully within the proximal ulna prior to IM screw tightening so that hoop stresses are minimized as the implant is advanced.
Once the body component is seated, a drill guide is used to determine the location of the holes to be drilled in the ulna. The drill guide is screwed to the ulnar body component with the posterior flange screw.
K-wires are placed through the drill guide to identify the location of where the cross locking drill holes need to be placed.
The groove within the polyethylene allows for the 7 degree valgus alignment that is necessary for maintenance of the carrying angle. Ultra-high-molecular-weight polyethylene (UHMWPE) is a high-modulus polyethylene with extremely long chains. The longer chain serves to transfer load more effectively to the polymer backbone by strengthening intermolecular interactions. This results in a very tough material with one of the highest impact strengths of any thermoplastic material.
A small cross is engraved in the polyethylene so as to line up with the cross on the posterior flange. Matching the cross in the polyethylene component with the flange facilitates implantation.
The cross locking screws are designed to mate with the posterior flange screw so that the posterior flange screw cannot loosen.
The proximal cross locking screws are then advanced. The screws are placed in opposing directions so that each side has a nut. The ligament reconstruction limbs are passed under the plates prior to compressing the plates by tightening the screw and nuts.
