Summary
PCL balancing in total knee arthroplasty (TKA) is diffi- cult to achieve with a bone-referenced technique. With a newly developed dynamic PCL spacer the flexion gap size of the knee, anterior tibia translation and distraction force were measured.We found that the flexion gap of the knee is a dynamic space which increased in size from an average of 17.3 mm with 100 N tension to 20.5 mm with 200 N tension. The anterior translation increased from 0.6 mm to 4.3 mm. The ratio between increase of the flex- ion gap and anterior tibia translation was 1:2. Total knee arthroplasty with the dynamic PCL spacer provided nor- mal stability in the anteroposterior direction, a good range of flexion, and a correct contact position of the femur onto the posterior 55%-60% of the tibia in patients operated on with this technique.
Introduction
Total knee arthroplasty (TKA) with retention of the pos- terior cruciate ligament (PCL) is an operation with excel- lent long-term results [1]. However, complications may occur if the PCL is not adequately balanced. A loose PCL may cause instability and pain [2-4] and a tight PCL may cause limited flexion,high polyethylene stresses,and wear [4, 5].
In recent studies of the normal knee with MRI the me- dial femoral condyle was shown to act as a circle that does not move anteroposteriorly in a broad band of motion.
There is no roll-back from 10° to 120° of flexion. The contact point on the tibia is maintained at approximately 2-5 mm posterior from the middle of the medial tibia plateau, which is at 54%-60% of the anteroposterior di- ameter of the medial tibia [6,7].The medial compartment is thus comparatively constrained. This contact point of the medial condyle should be restored during the knee implantation in order to restore normal kinematics.
In the knee the PCL is the main restraint to posterior translation of the tibia at greater flexion angles. The PCL is tensioned only in flexion and its main function is to
prevent posterior tibial subluxation [8]. As such, the PCL controls the contact point of the medial femoral condyle on the tibia from approximately 60° to 120° of flexion.
The goal of our newly developed PCL-balancing tech- nique is to achieve the correct PCL tension and with this correct pretension to define the optimal bone cuts to create an adequate flexion space. Fulfilling these two pre- requisites will automatically provide a normal contact point of the medial femoral condyle on the tibia plateau with subsequent good flexion and normal anteroposteri- or stability of the knee in flexion. A dynamic PCL spacer, the ‘’BalanSys PCL Tensioner’’,was developed by two of us (BC and UW).
This chapter describes the ligament-guided PCL balancing technique with a dynamic PCL spacer and the dynamics of the flexion gap size in relation to PCL tension and anterior tibial translation.The first clinical results with this operation technique are presented, together with kinematic data from laboratory experiments sup- porting the new approach of PCL balancing.
PCL-Balancing Technique with the Dynamic PCL Tensioner
The principle of the technique is based on creation of the correct amount of flexion space for the prosthesis with a pretensioned PCL. The PCL is not released; rather, addi- tional bone cuts of a few millimeters are performed or the thickness of the polyethylene is adjusted. The PCL is balanced only after the medial and lateral ligaments are balanced in extension and in 90º of flexion and after a rectangular flexion gap is created.
The BalanSys TKA is a ligament-guided knee system which uses a double-spring tensioner. The operative tech- nique is performed with the ‘’tibia-cut-first’’ technique with which the tibia cut and the ventral and dorsal femur cuts are made first and the extension cut of the femur is based on the polyethylene size chosen for the flexion space.
Following the tibia osteotomy of 6-8 mm with 7° pos- terior slope and preservation of the PCL insertion, the necessary medial or lateral releases are made in extension
28 Posterior Cruciate Ligament
Balancing in Total Knee Arthroplasty with a Dynamic PCL Spacer
A. B. Wymenga, B. Christen, U. Wehrli
with a tensor in the knee joint exerting 150-200 N tension.
An intramedullary femur guide is used to indicate the di- rection of the femur cut. Following the releases in exten- sion,the tibia cut should be parallel to the plane of the dis- tal femur cut. This indicates a correct mechanical axis, and no further releases are performed.
Thereafter the knee is flexed and a double tensor is in- serted with 100-150 N. The femur finds its rotational po- sition through the ligament tension of the central PCL and the collateral ligaments. The femur cutting guide is inserted over an intramedullary rod and rotated parallel to the tibia cut, creating a rectangular flexion space. The anterior and posterior femur osteotomies are performed.
The size of the femoral component is matched with the original medial femoral condyle dimensions, and ap- proximately 9 mm (prosthesis thickness) is cut from the posterior femoral condyle.This leaves the joint line on the medial side of the knee at the same level.
Now the PCL is balanced with the BalanSys PCL Ten- sioner with an adjustable spacer block. On the adjustable spacer the corresponding polyethylene sizes are marked 8, 10.5,13,and 15.5 mm.Thickness of the tibial tray (2 mm) and femoral component (9 mm) of the knee system are included. When the PCL tensioner is opened the PCL spacer increases in size.The surgeon can read on the han- dle of the device how much tension is being applied (scale from 0 to 250 N). On the femur side a sliding plateau is
mounted which enables anterior tibia translation when the spacer is gradually opened. Since the fiber course of the PCL is oblique from tibia inferior-posterior to the me- dial femoral condyle superior-anterior,the opening of the joint space with a tensor causes the tibia to move forward when the PCL is tensioned (⊡ Figs. 28-1, 28-2).
At the beginning of the tensioning by the BalanSys PCL Tensioner the joint opens in a proximal-distal direc- tion without translation as the PCL is not tensioned. As soon as the PCL is tensioned, the oblique fibers will pull the tibia forward. We currently use a 2- to 3-mm anterior translation for the correct pretension of the PCL. The rel- ative positions of femur and tibia with the spacer in situ are accepted as the correct position.
Now the surgeon has to check whether the prosthesis can be fitted into the created flexion space.
▬ If the indicated PE thickness on the BalanSys PCL Tensioner is 8 mm, this is accepted.
▬ If the flexion space is too small (e.g., 5 mm PE) the flexion space should be enlarged. The surgeon can choose to make an additional tibia cut of 3 mm or to use a smaller femur size (increments of 3 mm). After this additional cut is made, the flexion space can har- bor the tibia tray, the 8 mm PE, and the femur com- ponent and, given the volume of the prosthesis mate- rial, the PCL is automatically pretensioned after im- plantation of the material.
⊡ Fig. 28-1a-d.Tensioning the PCL during surgery with the BalanSys PCL Tensioner. a Without tension; b with tension. In c the anterior translation can be read, and in d the gap size can be read
a b
c d
▬ If the flexion space is larger than 8 mm the surgeon can choose a larger PE size. If the flexion space size is between two PE sizes (e.g., 9 mm) a larger size (10.5 mm) can be chosen but an additional 1.5-mm bone cut has to be made.
After the PCL balancing is finished the extension cut on the distal femur is made, guided by a tensor with 150 N and anticipating the same polyethylene thickness as cho- sen in flexion. If a bone-referenced technique is used and all bone cuts are made before flexion-space testing, the adjustment possibilities are more limited. With a tight flexion space and a correct extension space the surgeon
extension. In case of a loose flexion space a larger poly- ethylene size can be used with a re-cut of the distal femur.
Results
Flexion Gap Dynamics Measured with the BalanSys PCL Tensioner
In a prospective study at the Department of Orthopedic Surgery, Spital Bern Ziegler, 82 patients received a total knee arthroplasty with a PCL-sparing technique.The Bal- anSys PCL Tensioner was used to balance the PCL and create the correct flexion space.
The size of the flexion gap was measured with 100 N, 150 N, and 200 N. Also the anterior translation of the tib- ia was measured with the spacer. The results are summa- rized in ⊡ Table 28-1.
There is a large variation in the amount of opening of the flexion space and the applied tension. In some pa- tients the flexion space opens up to 14 mm with 200 N, whereas in others the joint space is only 6.5 mm.This may depend on the amount of tibia resection, the pre-existing laxity and morphotype of the patient, and the angulation of the PCL fibers, which may run a more vertical or hori- zontal course in some patients. Also, the weight of the extremity has an influence (⊡ Figure 28-3).
28
a b
⊡ Fig. 28-2a, b. Orientation of the PCL and relative movements of the femur onto the tibia. As the flexion gap increases by applied tension, the tibia is pulled anteriorly by the PCL
16,0 14,0 12,0 10,0 8,0 6,0 4,0 2,0 0,0
Flexion Gap size
0,0 2,0 4,0 6,0 8,0 10,0 12,0
Anterior translation
y = 0,5046x + 7,3216
⊡ Fig. 28-3.Relation between the increase of the flexion gap (mm) and anterior translation (mm) of the tibia with use of the BalanSys PCL Tensioner measured from 0 to 200 N tension
⊡ Table 28-1.Relation of flexion gap, tension, and anterior tibial translation
Tension BalanSys PCL tensioner 100 N 150 N 200 N
Flexion gap size in mm1(SD, range) 6.3 (0.6, 5.5-10.0) 7.6 (1.1, 5.0-11.0) 9.5 (1.8, 6.5-14.0) Anterior translation in mm (SD, range) 0.6 (0.6, 0.0-2.5) 2.2 (1.7, 0.0-8.5) 4.3 (2.0, 0.0-10.0)
1 Flexion gap size expressed in mm polyethylene; for real flexion gap size 2 mm tibia tray and 9 mm femoral component thickness should be added.
More important, however, is the anterior translation.
We found on average a 1:2 ratio between the increase of the joint space and the anterior translation when the ten- sion was increased from to 0 to 200 N. There was some variation from knees having a 1:1.5 ratio to a more than 1:2 ratio,as can be seen in Fig.28-2.From these data it is clear that the flexion space of a knee with an intact PCL is not defined as a fixed space but rather as a space depending on the tension and the direction of the PCL fibers and the anteroposterior position of the tibia. The greater the PCL tension, the larger the space becomes and the more ante- rior translation of the tibia occurs.
It is also clear that small adjustments of the flexion space have a significant influence on the anteroposterior position of the femur. If, for instance, an additional 3-mm bone cut of the tibia is made, the contact position of the femur will change (based on the 1:2 ratio) 2 × 3 = 6 mm, which can be the difference between a too posterior and a correct contact position of the femur. For example, if a 2.5-mm-larger PE insert is used, the femur will move 5 mm posteriorly, which could make the knee too tight.
Anteroposterior Laxity Measured in 90°
of Flexion in Clinical Patients with a TKA Implanted with the PCL Tensioner
In a prospective study (results prepared for publication) at the Department of Orthopedic Surgery, Spital Bern Ziegler, 141 patients (37 male and 104 female) were treat- ed with a BalanSys TKA between May 2001 and May 2003.
Mean age of the patients was 71 years (49-89).All patients were treated for osteoarthrosis. In 54 patients with valgus knees a lateral approach and in 87 patients with a varus knee a medial approach was used. These patients were evaluated with the Rolimeter (Aircast Europe Ltd.) [9-12]
in order to measure anteroposterior laxity of the knee in 90° of flexion preoperatively and postoperatively in the operating room after implantation of the prosthesis. The results are shown in ⊡ Table 28-2.
In a subset of these patients (n=65) the average flexion after surgery was 117.5°, indicating that with an adequate PCL, balancing can give a very stable knee in flexion with an anteroposterior drawer of only 5 mm and, despite this, a good range of motion.
Kinematics of TKA with the BalanSys PCL Tensioner in Laboratory Experiments
In a laboratory experiment five TKAs were implanted in fresh-frozen knee specimens using the BalanSys PCL Tensioner (results prepared for publication). The proce- dures were supported by a navigation system (PRAXIM, Grenoble, France). A relatively conforming, fixed-bear- ing insert was tested and then, in the same specimen, an AP-gliding meniscal-bearing insert was tested. After recording of the knee kinematics with passive motion, the PCL was cut and kinematics were recorded with a non-functional PCL.To quantify AP translation of the fe- mur relative to the tibia (results in Fig. 28-4), the refer- ence point on the femur was chosen as the intersection
⊡ Table 28-2.Anteroposterior laxity measured with rolimeter in 90° of flexion
Measure Preoperative Postoperative
Median 6.7 5.0
Mean 7.0 5.2
SD 2.9 2.0
Range 0-17 2-10
FB with PCL FB without MB with MB without
Flexion (degrees) 4,0
2,0 0,0 -2,0 -4,0 -6,0 -8,0 -10,0 -12,0
Anteroposterior position
0 15 30 45 60 75 90 105 120
⊡ Fig. 28-4.Average (n=5, mean +/-1 SEM) femoral AP translation of five knee specimens with a mobile- (MB) and a fixed-bearing (FB) TKP both before and after resection of the PCL
femoral component.
The fixed- and meniscal-bearing configurations show a correct contact point posterior to the middle of the tibial plateau in AP direction, even in extension (⊡ Fig.
28-4). In extension the meniscal variant is 2 mm anterior compared with the fixed-bearing variant. In flexion both bearing types resulted in roll-back between 60° and 120°
of flexion with a correct contact point similar to the nor- mal knee in 90° of flexion [6].
After the PCL has been cut the kinematics are dis- turbed and the femur contact point moves forward. The effects for the fixed-bearing insert are limited, whereas the effects for the AP-gliding meniscal-bearing insert are quite considerable. These data illustrate the effectiveness of PCL balancing with the BalanSys PCL Tensioner and confirm the importance of a well-functioning PCL, espe- cially in AP-gliding meniscal-bearing knees.
Discussion
A new concept of flexion-gap balancing with the Balan- Sys PCL Tensioner was developed. With the use of this dynamic spacer,the PCL can be adequately tensioned; the required amount of flexion gap for the prosthesis can be precisely determined and, if necessary, adjusted with small bone cuts or an increase of PE size. Patients from a case series operated on with this technique had an average of 117.5° of flexion with normal anteroposterior laxity, indicating a functional PCL.
The flexion gap is dependent on PCL tension and the anterior translation of the tibia. A small increase of the flexion space causes a comparatively large anterior trans- lation of the tibia with a ratio of 1:2. This is caused by the obliquely oriented PCL fibers that pull the tibia anterior with the opening of the flexion gap.A few millimeters dif- ference of bone resection can change the contact point of the femur on the tibia considerably. If the flexion gap is accepted without sufficient tension of the PCL the femur automatically slides forward. If the PCL is too tight the contact position is too posterior.
These findings may explain why it was difficult in some laboratory experiments to achieve normal PCL function with a PCL-retaining TKA [13]. Mahoney et al.
[14] found a normal PCL strain in only 37% after TKA.
They also concluded that the PCL strain was increased by 50% by inserting a 2.5-mm-thicker polyethylene insert.
Most [15] and Sorger [16], however, were able to balance PCL and the flexion gap in laboratory tests and to restore normal ‘roll-back’. Li [17] found a partial restoration of the roll-back beyond 60° of flexion. If the PCL was cut he found reciprocal anterior translation, showing the im- portance of the PCL.
by measuring the contact point of the femur on the tibia in flexion and AP-laxity in flexion. Misra [18] compared PCL resection and retention with a fixed-bearing condy- lar prosthesis and found roll-back in only 20% of both groups, indicating that PCL balancing was not achieved.
Matsuda [19] achieved only good anteroposterior stabil- ity in half of the patients with a PCL-retaining prosthesis.
Dejour [20] found anteroposterior instability in 41% of the patients with posterior cruciate-retaining (PCR) TKA, indicating a non-functioning PCL. Straw [21] found an- teroposterior laxity in 54% of patients with PCR TKA.
Kim [22], however, was able to achieve a contact point of 55% with PCR TKA, which is similar to that in the normal knee [6]. In mobile-bearing knees PCL balancing seems to be even more critical. Morberg [3] found a high rate of failures after AP-gliding mobile-bearing total knee arthroplasty due to flexion instability caused by inade- quate PCL balancing. Archibeck [23] found a low rate of clinical posterior instability and measured an average contact point of 45% (22%-98%) in meniscal-bearing knees. This contact point shows that the PCL balancing was not perfect in all patients. Hartford [24] found ante- rior sliding of the meniscal bearing in 70% of patients,in- dicating a non-functional PCL; the patients with roll-back had better knee scores and better flexion. Anteroposteri- or-gliding mobile bearings and flat fixed bearings depend more on soft-tissue restraints, whereas more fixed con- forming designs are more self-stabilizing [25]. Also with fluoroscopic analysis in vivo in PCR TKAs frequently show a paradoxical anterior sliding in flexion [26, 27], but results seem to vary between individual surgeons [26,28].
Bertin found a consistent posterior contact point in PCR TKA [29].Better flexion was found in patients with a pos- terior contact position.
From these studies it is clear that PCL balancing and the creation of a normal contact point of the femur on the tibia are difficult to achieve in PCL-retaining TKA with a bone-referenced technique. The flexion gap is not a stat- ic space but a dynamic space that is controlled mainly by the oblique PCL fibers that link flexion gap size and tibia translation in a 1:2 ratio. With the newly developed dy- namic PCL spacer it is possible to adequately balance the PCL in a reproducible way.
Conclusions
A ligament-guided PCL-balancing technique was devel- oped with a new device, the BalanSys PCL Tensioner. This dynamic PCL spacer controls the PCL tension and also determines exactly the required flexion space size.
Changes in flexion space have a large effect on anterior translation of the tibia, which may explain the difficulty
28
with PCL balancing in the past. The flexion space is ad- justed with small 1- to 2-mm bone cuts and the PCL is not released. We were able to achieve a high rate of antero- posterior stability with an average ROM of 117.5° in patients operated on with this technique, indicating a functional PCL.Laboratory tests confirmed normal kine- matics and normal contact points with the new PCL- balancing technique.
Acknowledgements.We acknowledge the co-authorship of A. J. Schuster and S. R. Thomann (Department of Orthopedic Surgery, Spital Bern Ziegler, Switzerland), T.
Wyss (Balgrist Spital, Zurich, Switzerland) and W. Jacobs (St. Maartenskliniek, Nijmegen, The Netherlands).
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