CLINICAL ORTHOPAEDICS AND RELATED RESEARCH
`Number 354, pp 49w56
`‘
`© 1998 LippincottWilliams & Wilkins
`
`Computer Assisted Knee Replacement
`
`Scott L. Delp, PhD*; S. David Stulberg, MD”; Brian Davies, PM) 7“;
`Frederic Picard, MD7‘7"; and Francois Leitner, PhD/l
`
`Accurate alignment of knee implants is essen-
`tial for the success of total knee replacement.
`Although mechanical alignment guides have
`been designed to improve alignment accuracy,
`- there are several fundamental limitations of this
`technology that will inhibit additional improve-
`ments. Various computer assisted techniques
`have been developed to examine the potential to
`install knee implants more accurately and con-
`sistently than can be done with mechanical
`guides. For example, computer integrated in-
`strumentation incorporates highly accurate
`measurement devices to locate joint centers,
`track surgical tools, and align prosthetic com-
`ponents. Image guided knee replacement pro-
`vides a three-dimensional preoperative plan
`that guides the placement of the cutting blocks
`and prosthetic components. Robot assisted knee
`
`From the *Departmcnts of Biomedical Engineering and
`Physical Medicine and Rehabilitation, Northwestern
`University, Chicago, IL; *Rehabilitation Institute of
`Chicago, Chicago, IL; **Department of Orthopaedic
`Surgery, Northwestern University Medical School,
`Chicago, IL;
`i‘Department of Mechanical Engineering,
`Imperial College, London, United Kingdom; iTDepart—
`ment of Orthopaedic Surgery, Grenoble South Hospital,
`Grenoble, France; and “PRAXIM Company, Imaging,
`Modeling and Cognition Techniques Laboratory, Uni~
`versity of Grenoble, Grenoble, France,
`Funding for the project on computer integrated instru-
`mentation was provided by the European Community,
`within the IGOS (Image Guided Orthopaedic Surgery)
`project and. by the Aesculap Co; Funding for the project
`on image guided knee replacement was provided by
`Computer-Assisted Surgical Technologies, Inc, Implex
`Corporation, and MusculoGraphics Inc; and funding
`for the project on robot assisted knee replacement was
`provided by the Department of Health in the United
`Kingdom as part of a DTI Link funding.
`Reprint requests to Scott L. Delp, PhD, Rehabilitation
`Institute of Chicago, Room 1406, 345 East Superior
`Street, Chicago, IL 60611.
`
`replacement allows one to machine bones accu-
`rately without
`the use of standard cutting
`blocks. The rationale for the development of
`computer assisted knee replacement systems is
`presented, the operation of several different sys~
`terns is described, the advantages and disadvan-
`tages of different approaches are discussed, and
`
`areas for future research are suggested. .
`
`Total knee replacement is widely used to re-
`lieve pain and improve function in patients
`with degenerative joint disease. Although total
`knee replacement is generally successful, fail—
`ures from component
`loosening,
`instability,
`dislocation, fracture, or infection occur in ap~
`proximately 5% to 8% of casesmw20 Less se~
`vere complications, such as patellofemoral
`pain or limited flexion, also contribute to sub-
`optimal outcomes in 20% to 40% of casesfifitl3
`The success of total knee replacement de-
`pends on several factors, including patient
`selection, prosthesis design, soft tissue bal—
`ancing, and alignment of the limb. Proper ro-
`tational and translational alignment of the
`prosthetic components "and of the limb are
`important factors that influence the outcome
`of knee replacement?) Incorrect positioning
`or orientation of implants can lead to accel-
`erated wear, component loosening, and de—
`graded functional performance%21
`An error in alignment of the components in
`any of the anatomic planes can have a detri-
`mental effect. Abnormal varus or valgus
`alignment has been reported to be a major
`cause of implant loosening.14 Rotation of the
`femoral and tibial components has a strong in-
`fluence on patcllar tracking, and malrotation
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`50
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`Delp et al
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`Clinical Orthopaedics
`and Related Research
`
`of the components can lead to patellofemoral
`complications.3 Alteration of the joint line has
`been associated with degraded postoperative
`function.9 Even a small (2.5 mm) anteroposte-
`rior displacement of the femoral component
`has been shown to alter knee range of motion
`(ROM) as much as 20".10 Posterior tilting of
`the tibial component also can affect knee
`ROM and tibiofemoral kinematics.7~15s19
`
`Mechanical alignment guides have im~
`proved the precision with which implants can
`be installed and are now one of the most impor—
`tant features that differentiates implant systems.
`As a result, major investments have been made
`in the development of new mechanical instru~
`ments during the last 2 decades. Although me-
`chanical alignment systems have been refined,
`errors in surgical alignment still occur. Teter et
`all8 reported that 8% of tibial cuts were .
`malaligned by more than 4‘? in the coronal plane
`when an extramedullary alignment guide was
`used. 'Even when using state of the art
`in—
`tramedullary alignment systems, surgeons find
`that it is difficult to install knee implants within
`2° to 3° varus or valgus alignment.l6 Other de—
`grees of freedom, such as rotation of the
`femoral or tibial components, are even less re~
`peatable than varus or valgus alignment.
`There are fundamental limitations of me—
`
`chanical alignment systems that inhibit addi-
`tional improvements. For example, in most
`mechanical alignment systems some degrees
`of freedom, such as rotationof the femoral
`and tibial components, and positioning of the
`patellar component, are aligned by visual in-
`spection. Other degrees of freedom are refer—
`enced to external jigs, which are difficult to
`position consistently relative to the bones. In
`general,
`alignment guides
`are designed
`based on standardized bone geometry; opti—
`mal placement of the components may not be
`achieved when the patient’s bones differ
`from the bone geometry that was assumed by
`the instrument designer.
`'
`Three types of computer based systems
`recently have been developed to overcome
`the problems with mechanical alignment
`systems. The first type, computer integrated
`
`instru~
`instruments, augments mechanical
`ments through the addition of measurement
`probes that can be used to locate joint cen»
`ters, track surgical tools, and align prosthetic
`components. The second type, image guided
`knee replacement, provides a three-dimen-
`sional preoperative plan that guides ' the
`placement of the components. The third
`type, which uses active robotic devices, al—
`lows one to make highly accurate resections
`without the use of standard cutting guides.
`Most computer assisted knee replacement
`systems exist as laboratory prototypes,
`al—
`though some have been tested in the operat—
`ing room. These initial
`tests suggest
`that
`computer assisted knee replacement will
`play an important role in the evolution of
`knee arthroplasty. The purpose of this article
`is to describe a range of alternatives to me»
`chanical
`instruments, discuss the potential
`advantages and disadvantages of computer
`integrated systems, and suggest areas for fu-
`ture research and development.
`
`COM?UTER INTEGRATED
`INSTRUMENTS FOR KNEE
`
`_ REPLACEMENT
`
`instru-
`The capabilities of mechanical
`ments can be enhanced by integrating them
`with highly accurate measurement equip- '
`ment. To determine the advantages of this
`approach, computer software was developed
`that uses measurements from an optical lo-
`calizer (Optotrack, Northern Digital, Water—
`loo, Ontario, Canada) to guide the placement
`of the cutting guides for Aesculap knee im-
`plants (Fig l). The localizer measures the
`three—dimensional coordinates of light emit-
`ting diodes with an accuracy of 0.1 mm. Sets
`of four to six light emitting diodes were
`mounted into fixtures to create reference
`frames. These reference frames can be at-
`tached to the bones and to the surgical instru-
`ments to track the positions and orientations
`of each surgical tool relative to the bones.
`The use of computer integrated instru~
`ments introduces two novel stages to the sur—
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`Number 854
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`Computer Assisted Knee Replacement
`
`51
`
`
`
`Fig 1. An optical localizer (left
`frame) is used to monitor the
`position and orientation of ar-
`rays of
`light emitting diodes
`fixed to the femur,
`tibia, and
`cutting guides (lower frame).
`The" computer provides visual
`feedback (right frame) so that '
`the user can position and ori—
`ent the tibial cutting guide. A
`computer is controlled by afoot
`switch.
`
`gical procedure. The first stage determines the
`mechanical axes of the femur and tibia. Refer-
`
`ence frames are fixed to the iliac crest, the dis-
`
`plant was 90° in five cases, 88° in one case,
`and 87° in one case. The angle between the
`mechanical axis of the tibia and the tibial
`
`tal femur, the proximal tibia, and the foot with
`custom designed screws. The hip is rotated
`through a ROM, and the position and orienta—
`tion of the reference frames fixed to the pelvis
`and femur are used to locate the hip center. In
`a similar procedure, the knee and ankle are
`manipulated to locate the average centers of
`these joints. The mechanical axis of the femur
`is calculated as the axis from the hip center to
`the knee center. The mechanical axis of the
`tibia is calculated as the axis from the ankle
`center to the knee center.
`.
`
`In the second stage of the procedure the sur—
`geon secures reference frames to the cutting
`blocks. The computer workstation displays the
`position of the cutting block relative to the de—
`siredposition (that is, orthogonal to the me-
`chanical axis of the bone). Once a jig is ori—
`ented properly it is secured in position and the
`cuts are made with a standard oscillating saw.
`Knee implants were installed in seven ca—
`davers to test this system. These initial ex—
`periments showed that the system was easy
`to use, required minimal preoperative imag—
`ing, and did not extend the time of operation.
`Radiographie measurements taken after the
`installation of the implants showed that the
`angle between the mechanical axis of the fe—
`mur and the distal plane of the femoral im—
`
`component was 90° in all cases.
`. From January to May 1997 the system
`was used to install implants in four patients.
`There were no complications and the aver—
`age tourniquet time and postoperative bleed—
`ing were less than for standard knee replace—
`ment. Analysis of postoperative radiographs
`also were encouraging (Table 1). The system
`described in this section was developed at
`the PRAXIM Company, Grenoble, France,
`and the Department of Orthopaedic Surgery,
`Grenoble South Hospital, Grenoble, France.
`
`IMAGE GUIDED KNEE
`REPLACEMENT
`
`Image guided knee replacement begins with
`preoperative planning. To create the preopera—
`tive plan, three-dimensional computer models
`of the patient’s femur and tibia are constructed
`from computed tomographic (CT) data. Once
`the computer models of the bones have been
`created, planning software orients the tibial
`and femoral components and calculates bone
`resections that align the mechanical axis of the
`limb and produce the intended implant con-
`tact. An intraoperative system determines the
`position and orientation of the patient’s femur
`and tibia and guides the placement of the cut—
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`52
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`Delp et al
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`Clinical Orthopaedics
`and Related Research
`
`TABLE 1. Comparison Between Computer Assisted Technique and Conventional
`SurgeryWW
`
`Conventional.
`Computer Assisted
`Measured Parameter
`Surgery (n = 65)
`Technique (n = 4)
`
`Tournlquet time (minutes)
`Postoperative bleeding (mL)
`Tibiofemoral angle* (degrees)
`
`365 (240—590)
`181(173—181-183—184)
`
`109
`
`618 (25—1440)
`Varus (65%) 183
`Valgus (26%) 177
`Normal (9%) 180
`Varus (50%) 92
`Valgus (26%) 88
`Normal (24%) 90
`Varus (63%) 92
`Valgus (25%) 88
`Normal (12%) 90
`
`Femoral angle” (degrees)
`
`88.2 (86—88-89-90)
`
`Tibial angler (degrees)
`
`91.5 (90-90-9193)
`
`* Tibiofemoral angle is defined as the angle between the epicondylar axis and the tibial implant plateau.
`** Femoral angle is the angle between the femoral mechanical axis and the epicondylar axis.
`1‘ Tibial angle is the angle between the tibial mechanical axis and the tibial implant plateau,
`
`ting jigs onto the bones so that the resections
`determined in the preoperative plan can be
`made. At the end of the operation, the differ—
`) ence between the preoperative plan and the ac~
`tual surgery can be measured.-
`" An imaging protocol was developed in
`which 10 images in the transverse plane,
`spaced ’5‘ mm apart, are taken at the hip and
`ankle. Approximately 100 images, spaced at
`*1 mm, are taken to define the articular sur-
`faces of the knee. Surgical planning software
`uses a Canny edge filter5 to locate the bone
`boundaries in the planar CT images. Three—
`dimensional models of the articular surfaces
`
`of the knee are then created (Fig 2). The cen»
`ters of the hip, knee, and ankle are located in
`the CT images to determine the mechanical
`axis of the limb. The epicondylar axis of the
`femur and other key points also are located
`in the image data. Based on these measure—
`ments, the planning software calculates the
`implant size, the implant positions, and the
`bone resections that align the limb and pro—
`duce the intended implant contact. The im~
`plant size, position, and orientation also can
`be changed by the user.
`The intraoperative system consists of a
`graphics workstation, a coordinate measure—
`ment probe, and a set of cutting blocks that
`
`have been modified to attach to themeasure—
`
`ment probe, similar to the system shown in
`Figure 1. The intraoperative system is used
`to determine the position and orientation of
`the patient’s femur and tibia and to guide the
`surgeon in the placement of the cutting jigs
`'so that the resections specified in the preop—
`erative plan can be made.
`One of the key steps in the operation is reg—
`istration. Registration is the processof deter-
`mining the geometric correspondence between
`the surgical plan and the patient’s bones. Reg—
`istration is accomplished by a two-step proce-
`dure. In the first step, the computer displays a
`suggested position and orientation of the mea-
`surement probe with respect to the femur or
`tibia. The user attempts to align the measure—
`ment probe as displayed. The system. assumes
`that the user has aligned the probe exactly as
`displayed and computes the geometric trans-
`formation for this initial registration. The ini—
`tial registration then is refined in a second reg~
`istration phase. In this phase, the user samples
`a set of 20 to 25 points distributed over the sur-
`face of the bone with the measurement probe.
`Given this set of sampled points (Si), an itera-
`tive closest point algorithm,4 finds the rotation
`(R) and the translation (T) that minimizes the
`mean squared distance between the sampled
`
`'
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`7 Number 354
`September, 1998
`Computer Assisted Knee Replacement . 53
`
` Fig 2. Preoperative plan for image guided knee
`
`
`
`replacement. Computed tomographic data is
`used to create a three-dimensional model of the
`femur (upper frame), which then is used to plan
`the placement of the implants and the cutting
`blocks (lower frame).
`
`
`
`points and the set of closest points on the com—
`puter model of the bone surface (Mi). That is,
`R and T are determined that minimize the
`function:
`i
`
`.1 N.
`
`f(R.T):Ns§1“Mr— (RSz+T) II‘2
`
`where Ns is the number of sampled points.
`Tests were cenducted to quantify the an-
`gular error introduced by the registration
`process. After implanting markers into the
`bones of a cadaver, CT images were acquired
`according to the protocol defined above. The
`markers were ceramic spheres with a diame—
`ter of 1 cm, which were mounted on delrin
`posts. The centroids of the markers were lo—
`cated in the image data along with the sur-
`faces of the bones.
`
`The spheric markers Were located in the
`laboratory with a measurement probe by sam-
`
`pling 15 to 20 points on the surface of each
`sphere. A registration between the computer
`model and the bones was determined from the
`
`locations of the spheres; this was considered
`to be the true registration. A registration then
`was performed according to the two-step pro—
`cedure that uses 20 to 25 points sampled from
`the surface of each bone.
`
`These tests showed that the average error
`introduced by the registration was less than
`10 for all angles, except tibial rotation (Table
`2). The maximum error in 20 trials was 5.8°
`in one instance for tibial rotation. This oc—
`
`curred because the set of points sampled
`from the tibial surface did not adequately de-
`fine tibial rotation.
`
`An evaluation with 10 surgeons analyzed
`the planning software, the intraoperative guid-
`ance system, and the customized cutting
`blocks. Nine of the 10 surgeons reported that
`they thought that the system was easy to use
`and was capable of improving their accuracy.
`The system described in this section was de»
`veloped by Peter Loan, MS, Craig Robinson,
`BS, and Arthur Wong, MS, of MusculoGraph—
`ics Inc, Evanston, IL, in collaboration with
`Scott Delp, PhD, and David Stulberg, MD.
`
`ROBOT ASSISTED KNEE
`REPLACEMENT
`
`Several groups have implemented prototypes
`using industrial robots to improve the accu—
`racy and precision of bone resections. Matsen
`
`TABLE 2. Registration Errors in Image
`Guided Knee Replacement
`
`Average Maximum*
`
`Angle
`Error (°)
`. Error (°)
`
`Femur
`
`0.4
`0.7
`0.7
`
`Varus or valgus
`Fiexion or extension
`Rotation
`Tibia
`1.9
`0.4
`Varus or valgus
`2.0
`0.9
`Fiexion or extension
`
`
`2.7Rotation 5.8
`
`0.8
`2.8
`2.8
`
`* Maximum in the 20 trials.
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`54
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`Delp et al
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`et al12 implemented a system for distal femoral
`arthroplasty and compared the accuracy of
`their laboratory prototype with conventional
`techniques. Their tests on plastic bone models
`showed that 14 of 30 cuts made with mechani-
`
`cal alignment guides had errors greater than
`2°. Only six of 30 cuts made with robotic as—
`sistance deviated from the desired values by
`more than 2°. KienZle et al11 developed surgi—
`cal planning software and a robotic procedure
`to drill the alignment holes for conventional
`cutting blocks for femoral and tibial implants,
`Robot assistance also provides the capabil~
`ity to machine bone [surfaces
`to avoid
`malalignment implicit in the cement mantle
`and to promote bone ingrowth attributable to
`a larger contact area between the prosthesis
`and the bone. Fadda et al8 have linked a so:
`phisticated preoperative planning system to a
`standard industrial robot to allow placement
`of cutting blocks and machining of the bones.
`These prototypes haVe used standard in—
`dustrial robots, typically a Puma robot (Uni—
`mation Inc, Pittsburgh, PA). Although the
`tests of systems that incorporate industrial
`robots have showed some possible advan—
`tages,
`these robotic devices were not de—
`signed for use in the operating room.
`Davies et a]6 have designed a special pur—
`pose robot for knee surgery (Fig 3). Labora-
`tory studies of the robot performance in cut—
`ting animal bones show the overall accuracy
`of the imaging, modeling, registration, and re-
`bot cutting to be approximately 1.3 mm. This
`active constraint robot (ACROBOT) uses a
`force Controlled lever near the tip of the robot,
`The surgeon can hold the lever and move the
`robot under servoassistance within a pro—
`grammed region permitted by the robot. Ac—
`cess to regions that include ligaments, nerves,
`and vasculature is prevented by the robot. The
`robot also can provide a virtual cutting block
`that constrains a rotary cutter to cut various
`shapes, such as a series of planes suitable for
`mounting the implant. If the user attempts to
`machine beyond the permitted plane, the robot
`transitions into a high gain position servo, re—
`sisting the surgeon’s attempts to move into
`
`Clinical Orthopaedics
`and Related Research
`
`
`
`Fig 3. Photograph of the ACROBOT system.
`
`forbidden regions. Thus, the robot can provide
`accuracy and constraint while the surgeon can
`feel the forces exerted on the bone and use his
`or her judgment to slow down or take a lighter
`cut when a hard region of bone is encountered.
`This synergy betWeen the robot and surgeon
`keeps the surgeon in control of the procedure.
`To smoothly and accurately back drive the
`ACROBOT, while being able to feel
`the
`forces applied to the cutter, low and equal
`impedance is required from the robot. The
`ACROBOT therefore has been designed to
`provide pitch and yaw motions, and linear
`motions, which all have an equal and a low
`impedance. It is hoped that these features
`will produce a synergy betWeen the machine
`and the surgeon and enhance the acceptance
`of robotic systems in the future.
`The system described in this section was
`developed by the Mechatronics in Medicine
`Group in the Department of Mechanical Engi«
`neering at Imperial College, London, United
`Kingdom.
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`Number 854
`Computer Assisted Knee Replacement 55
`September, 1998
`
`
`
`DISCUSSION
`
`Computer integrated instruments that com—
`bine standard cutting guides with highly accu—
`rate measurement equipment are a natural ex—
`tension of current techniques and offer several
`potential advantages. The computer
`inte—
`grated instruments described here showed the
`potential to reduce surgical errors and ensure
`rapid, precise positioning of the cutting
`guides. This system also eliminates the need
`for intramedullary and extramedullary align—
`ment guides. Although there are new elements
`of the surgical procedure, several complex -
`alignment steps are eliminated. This system
`achieves these advantages without additional
`imaging or preoperative planning.
`However, there are several shortcomings of
`this system. First, the current method of locat—
`ing the hip center requires the installation of a
`screw into the iliac crest. Second, the system
`requires a computer and an optical tracking
`device, which are more expensive than me
`chanical alignment guides. Finally, although
`the computation of the mechanical axes of the
`femur and tibia can improve alignment accu—
`racy, not all of the degrees of freedom are con—
`trolled by the current alignment procedure.
`Image guided knee replacement
`is a
`greater departure from. current practice, but
`has some additional capabilities when com-
`pared with computer integrated instruments
`that do not include preoperative planning.
`For example, analysis of the three-dimenv
`sional image data allows one to determine
`the size of the implant preoperatively, potena
`tially reducing inventory. This system also
`provides guidance for all 6 degrees freedom
`of the implant. Image guided knee replace—
`ment allows one to measure actualvplace—
`ment of the implants relative to the planned
`placement. This provides the possibility to
`perform studies that compare the accuraCy of
`mechanical instruments to computer based
`instruments. It also provides the possibility to
`study the effects of alignment on implant
`_ wear and patient outcomes. Because the
`placement of the implants is guided by highly
`
`accurate tracking devices, image guided knee
`replacement potentially provides more accu-
`rate, reproducible installation of prosthetic
`components. Additional work is needed to test
`this hypothesis.
`The advantages of image guided knee re-
`placement do not come without a cost. For ex—
`ample, this approach requires a preoperative
`CT scan, which is not a standard part of knee
`replacement. Image guided surgery also re-
`quires the construction of a preoperative plan,
`which takes approximately 3 hours with the
`current system. Finally, all
`image guided
`surgery involves registration to determine the
`geometric correspondence between‘the surgi~
`cal plan and the patient. Registration can in-
`troduce errors and is the least reliable element
`
`of most image guided surgery systems.
`Similar to image guided knee replacement,
`robot assisted knee replacement has the ad-
`vantages (and disadvantages) of preoperative
`imaging, modeling, and planning. A robot,
`however, provides the capability to create a
`physical constraint, unlike a surgeon holding
`an image guided surgical tool. In addition, the
`machining capability of a robot may provide a
`more accurate fit between the prosthesis and
`the bone, making the use of cement unneces~
`sary in some cases. In addition to these gen-
`eral benefits of robot assisted surgery, the AC—
`ROBOT concept provides a synergy in which
`the robot provides accuracy and constraint
`and allows the surgeon to use his or her sense
`of touch to control the procedure.
`The cost associated with the use of a robot
`
`in the operating room is currently a major
`limitation of this technology. Current or—
`thopaedic robotic systems cost approxi—
`mately $500,000. Until the cost can be re—
`duced dramatically,
`robotic systems
`are
`unlikely to be used in a large number of hos~
`pitals. This has motivated the development
`of special purpose surgical robots with the
`goal of providing a tool that is simple and
`cost effective.
`
`The systems described here are in the pre—
`liminary stage of development, and the limita—
`tions of these systems suggest areas for future
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`56
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`Delp et al
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`Clinical Orthopaedics
`and Related Research
`
`research. For example, although soft tissue bal—
`ancing is one of the key factors that influences
`the success of total knee replacement, none of
`the current systems include a biomechanical
`analysis of the soft tissues in the surgical plan—
`ning process. Analysis of the interplay between
`implant design, surgical alignment, soft tissue
`tensions, and joint function should be included
`in future surgical planning softWare.
`, Although some surgeons see the use of pre—
`operative CT scans and imaging as an advan—
`tage, others see this as an extra and costly re-
`quirement. Future research aimed at reducing
`the reliance of the image guided and robotic
`techniques on preoperative CT scans may in—
`crease the acceptance of these approaches.
`Additional research is needed to test the ef—
`ficacy of computer assisted knee replacement
`systems. The measurement equipment incor-
`porated into these systems should be used to
`quantify the accuracy of current mechanical in—
`struments (in all anatomic planes) ande-to com—
`pare this with the accuracy of computer based
`systems. It is essential to. determine whether
`improvements in alignment accuracy improve
`functional outcomes, reduce loosening, and de~
`crease the need for revisions. If the accuracy
`and outcomes are improved with computer as—
`sisted techniques, then the introduction of new
`computer assisted procedures, including uni—
`compartmental knee replacement and high tib—
`ial osteotomy, would be warranted.
`
`References
`
`l. Aglietti P, Buzzi R: Posterior stabilized total condylar
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