MOST DIAGNOSTIC IMAGING IS NOT DYNAMIC. PHYSIOLOGY IS.

4DDI | Four Dimensional Digital Imaging

WHY DYNAMIC IMAGING?

Dynamic tissue balance remains the “Holly Grail” of joint replacement surgery

3D static knee alignment data obtained intraoperatively have limited capacity to explain the variance in functional outcome. Although alignment and component position can be precisely measured intraoperatively, dynamic tissue balance and other intrinsic patient factors remain dominant in determining the outcome. In the study of intraoperative navigation data was collected prospectively for 134 knees undergoing cemented, posterior-stabilized total knee arthroplasty (Benjamin J. Widmer et. al.). Computer-assisted navigation system (coronal alignment, ligament balance, range of motion, external tibiofemoral rotation) with 1-year outcomes (36-item Short-Form Health Survey, Oxford Knee Score, range of motion)

PRE-OPERATIVE PLANNING

Pre-op planning depends on 2D radiographs at 0 degrees F/E. All planar  two-dimensional (2D) projections have errors in true geometry  due to magnification, distortion, superimposition, and misrepresentation of structures (30-50% error).

Geometry Error

Use of 3D geometry data solves some of the above problems but still the patient is lying down during 3D imaging i.e. no physiological load is applied . It is clear that the passively loaded or unloaded knee gap information should not be used to align the prosthesis.

For the same reasons robotically driven installations are very precise systems that still depend on static 3D joint shape information input. Therefore, they suffer the same accuracy problems and imperfections, often delivering inappropriate bone cutting levels  and no tissue balancing feedback to the surgeon.

Example of 4DDI Use of Imaging to Improve Clinical Outcomes

The most important predictor of clinical outcome in total knee arthroplasty (TKA) is placement of the femoral-tibial components.

Performing a TKR depents on:

– The accurate execution of key bone cuts in the correct orientation to the appropriate axes.

-There fact that huge potential for cumulative errors to occur, which may have significant and dramatic effects on function and longevity

Joint reconstruction example of possible errors:

-Anatomical axis: entry point  selection and orientation of intramedullary rod-guides cutting block (F) and jigs (T).

20-30% ERROR

– Accurate rotation of the femoral component is based on accurate selection of  transepicondylar axis and  whiteside’s  line selection.

50% ERROR

Tissue balancing inaccuracies can lead to pain and poor function because…

– Insccurate alignment of the patellofemoral joint… because it depents on the rotation of the implanted femoral and tibial component of excessive internal rotation of the tibial component

– relative external rotation of the tibial tuberosity,

– increasing subluxation or dislocation of the patella  of excessive external rotation of the tibial component

– posterolateral overhang of the prosthesis with soft-tissue impingement and relative internal rotation of the tibial tuberosity.

Errors in computer navigation based systems

– Computer navigation relies on accurate data input in order to calculate the mechanical axis from the center of hip rotation, through the center of the knee, to the center of the ankle.

– It will not take into account variations in anatomy, such as a very bowed tibia or a pronounced femoral bow in the sagittal plane. In the latter situation it is possible to cause femoral notching.

THE AXODE

4DDI | Four Dimensional Digital Imaging

The axode is the loci of the  instantaneous  axes or rotation: This surface is traced by the instantaneous screw axes of the movement of a body. The instantaneous screw axis, or ‘instantaneous helical axis’ (IHA), is the axis of the helicoidal field generated by the velocities of every point in a moving body.

Knowledge of the articular surface  geometry, location of the axodes and the amount of sliding, gliding, rolling define the patient specific arthrokinematics.

This information has never benn availble to the surgeon-clinician before

Offer axode view for the study of patient-specific knee kinematics of the actively in-vivo loaded knee n different times of the support face of strenuous activities.

True 4D

4DDI | Four Dimensional Digital Imaging

In reality i.e. true 4D the 3D mechanical axes  change at the different stages of the range of motion. Note this is a loaded knee during walking.
True direct Arthro-Kinematics measurement of gait demonstrates that the knee is rotating, translating, rolling, sliding and gliding during the full range of motion

The Hinge Joint Assumption

4DDI | why dynamic

Is the current state of the art hypothesis in analyzing knee arthrokinematics pre-operatively, intra-operatively and postoperatively i.e. cancels out all realistic rolling, sliding and gliding during the full range of motion of the joint-leading to significant errors in joint gap, flexion-extension cuts, tibial cuts, recession estimates etc..

The True Joint 4D Gap

HINGE JOINT: ONE FLEXION/EXTENSION AXIS FOR THE WHOLE RANGE OF MOTION

Three examples of changing the radius of rotation of the knee:  which one is the correct?

Hinge Joint / Large radius of rotation

4DDI | why dynamic

Hinge Joint / Small radius of rotation

4DDI | why dynamic

4D Joint GAP / Appropriate tissue balancing

What is the error between the true gap  and the gap of a hinge joint assumption?

It is obvious that there are three different shapes and thickness for the space between the femur and tibia, depending upon assumptions made regarding rotation of the tibia about an axis, an axode, or the location of the axes.  Only one can be correct and physiologic: The true joint 4D gap.

TRUE GAP / HINGE JOINT GAP

4DDI | Four Dimensional Digital Imaging

True Flexion Extension Gaps

ERRORS IN ESTIMATION OF TRUE FLEXION EXTENSION GAPS CAN BE AS HIGH AS 30-50%

Estimated Error

Leading to shallow or deep cuts and inappropriate tissue balancing before closing

As further illustration that the assumption of a hinged knee joint can lead to error, the red areas shown in the gap analysis here indicate the discrepancy between a gap planned using a 2D hinge joint assumption versus that of a dynamic 4D analysis shown in red.

4DDI | Four Dimensional Digital Imaging

No need for Sizing jigs, alignment rods, gap spacers, tensors

When  true 4D gaps  is used to calculate the knee cuts, not only our technology will improve outcomes, but it can significantly improve the workflow over the entire process, from surgical planning to follow-up.  One of the key aspects in the workflow challenges is the simplification of the surgical procedure.  4DDI enables overall efficiency by shortening the procedure time and by eliminating the array of instrumentation that is time consuming and costly to employ.

FIND OUT NEXT HOW IS THIS POSSIBLE WITH 4DDI SOFTWARE  TECHONOGY AND DISPOSABLE CUTTING GUIDES