21Cruciate Deficiency in the ReplacedKnee 21 21

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Chapter 21 · Cruciate Deficiency in the Replaced Knee – J. Victor

21 Cruciate Deficiency in the Replaced Knee

J. Victor

Summary

At least one of the cruciate ligaments is sacrificed in most total knee arthroplasties.This can lead to important functional changes in the joint. In severe cases, instability causes early failure and revision of the implant. In a less extreme form, the cruciate deficiency leads to abnormal kinematics, affecting activities of daily life and reducing functional capacity of the knee joint. Technological advances opened the potential for studying the in vivo characteristics of the replaced knee. Significant differences in kinematics between the normal and the replaced knee have been reported in the literature. Many of these differences can be attributed to either anterior or posterior cruciate ligament deficiency following total knee arthroplasty.

Introduction

In the normal human knee, passive stability is provided by the ligaments, the menisci, and the congruency of the joint surfaces. The cruciate ligaments play an important role as stabilizers in the sagittal plane. The anterior cruciate ligament (ACL) is the primary restraint against anterior translation of the tibia relative to the femur. The posterior cruciate ligament (PCL) is the primary restraint against posterior translation of the tibia relative to the femur. In the horizontal plane both cruciate ligaments resist excessive internal rotation of the tibia relative to the femur. In the coronal plane, the PCL is a secondary restraint against valgus angulation of the tibia, relative to the femur. In addition to their stabilizing and proprio- ceptive function,the cruciate ligaments also play a crucial role in guiding the kinematics of the knee during the arc of motion, as their location, length, and insertion are closely related to the geometrical characteristics of the distal femur and the proximal tibia [21, 32].

When a total knee arthroplasty is inserted, much of this delicate mechanical system is altered. Today’s resur- facing-type knee prostheses mimic the condylar shape of the knee but are far from being truly anatomical. In addition, in most total knee arthroplasties the ACL is resect-

ed. Many types of total knees also require resection of the PCL. It is clear that this will alter the stability and the kinematics of the replaced knee in comparison to a healthy normal knee joint. Historically, the loss of primary and secondary stabilizers (cruciate ligaments and menisci) was recognized at an early stage by the design- ers of the total condylar knee. They attempted to restore the sagittal stability of the knee joint by creating a concave tibial surface that accommodated the femoral condyles.

Later on, this design was converted to the posterior stabilized knee to avoid the posterior kinematic conflict and still maintain sufficient stability [23].

Design Solutions for Obtaining Stability in the Sagittal Plane

There are basically two options for improving sagittal stability in the replaced knee without completely constrain- ing the joint as in a hinged design. One option is to alter the surface congruity, the other is to include a type of cam-and-post mechanism.Altering the surface congruity is often misinterpreted, as the terminology is misleading.

⊡ Fig. 21-1a, b. a An anterior build-up of the polyethylene insert pro- vides resistance against posterior translation of the tibia with respect to the femur and is a substitute for PCL function. b A posterior lip on the poly- ethylene insert provides resistance against anterior translation of the tibia with respect to the femur and is a substitute for ACL function

a b

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the femur, whereas a ‘posterior lip’ provides a buffer towards anterior translation of the tibia relative to the femur. In terms of sagittal stability, the ‘anterior build-up’

is an attempt to copy the PCL function (⊡ Fig. 21-1a). The

‘posterior lip’, on the other hand, is an attempt to copy ACL function (⊡ Fig. 21-1b).

The cam-and-post mechanism can provide a mechanical buttress against posterior translation of the tibia with respect to the femur.It should be understood,how- ever, that this cam-and-post mechanism is without function as long as there is no contact between the two components. More specifically, if there is increased anterior laxity of the tibia with respect to the femur (deficient ACL function),the initial anterior-directed force vector of the extensor mechanism can pull the tibia forward [33]

and disengage the cam-and-post mechanism (⊡ Fig. 21-2).

on the tibia. This can lead to anterior subluxation of the tibia and disengagement of the cam-post mechanism.

The point at which the cam and post engage during the flexion cycle depends upon the thickness of the patella- button complex, the geometry of the femoral component, the surface geometry of the tibial polyethylene, the position of the post and the cam, and the surgical posi- tioning of the components.

In some knees, contact between the cam and the post never occurs during the flexion cycle

⊡ Fig. 21-3a-d.Sagittal instability leading to polyethylene wear with anterior subluxation of the tibia (a). The synovial hypertrophy is conspicuous as a soft-tissue shadow on the lateral X-ray. The retrieved specimen (b) shows a wear pattern that is strikingly similar to the arthritic wear pattern (c) of the chronic ACL-deficient knee (d), suggesting that the mechanism of failure (sagittal instability) is similar

a

b

c

d

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In other words, in the absence of normal ACL function, the traditional cam-and-post mechanism can only pre- vent nonphysiological posterior translation, starting from a given position and in a given direction, and is not able to provide gradual stabilization of the joint during the full range of motion.

Instability as a Clinical Entity

Sagittal instability has been described as a clinical complication after TKA.The above-described mechanisms to im- prove functional sagittal stability can function only when the collateral ligaments provide sufficient stability during flexion. Their incapacity to do so is referred to as „flexion gap“ instability.This can lead to clinical symptoms such as reduced range of motion,pain,and giving way [7].If the instability is significant,dislocation of the knee can occur,depending upon the design of the cam-and-post mechanism [26]. The most notorious complication of sagittal instability is early polyethylene wear (⊡ Fig. 21-3).

Sharkey et al. [35] recently investigated the causes for revision total knee arthroplasty.Instability was the reason for early revision in 21% of cases and the reason for late revisions in 22%. The biomechanical explanation for the detrimental effect of repetitive sliding on the polyethylene was described by Blunn et al. [9]. There is further clinical evidence from outcome studies that looked at the results of polyethylene exchange for wear. Babis et al. [3]

reported a re-revision rate of these polyethylene ex- changes of 25% at a mean of 3 years. These findings were confirmed by Brooks et al. [11], who found a re-revision rate of 29% at less than 5 years. These data strongly suggest that the cause of failure is not intrinsic to the polyethylene but is related to the instability of the replaced knee.

Another potential consequence of sagittal instability is anterior knee pain.Excessive posterior laxity of the tibia increases the patellofemoral joint reaction force. The use of a posterior-stabilized knee arthroplasty has been advocated in case of prior patellectomy.

The Relation Between Sagittal

Instability and Kinematic Characteristics

All in vivo studies using fluoroscopy as a tool to investi- gate the kinematics of the knee after implantation of a total knee arthroplasty report abnormal kinematics as compared with the normal knee [2, 4, 5, 8, 13-16, 20, 22, 27, 31, 37-41, 44, 45]. These differences include less posterior lateral femoral roll-back as the knee moves from extension to flexion, abnormal axial rotation between the femur and the tibia, a different center of rotation of the knee in the horizontal plane, and condylar liftoff. The

reported results for the posterior cruciate ligament-retaining (CR) knees are consistently more variable than for the posterior-stabilized (PS) knees. Non-physiological roll-forward of the medial femoral condyle during flexion is a common finding. Roll-back on the lateral side during flexion is noted to a greater extent in the PS than in the CR devices. Overall, PS knees display greater roll-back and have a better range of motion [5, 6, 15, 20, 44, 45].

The role of the posterior cruciate ligament in total knee arthroplasty (TKA) has been discussed at large in the orthopedic literature.Arguments of theoretical order [19], functional order [1, 17, 36], and survivorship [18, 34, 46] have been used to prove the advantage of retention over substitution and vice versa. Other papers focused on the degree of deformity [28, 36] or the underlying cause of arthritis [29] as indications for resecting the PCL and using a PS design. Numerous authors are capa- ble of producing good clinical outcome results with either technique, CR or PS [10, 17, 30, 34, 42, 44, 45].A recent paper by Straw [42] shows equal results for patients with PCL-retaining total knee arthroplasty and PS knees.

Significantly worse results are reported for patients with a CR knee replacement where a tight PCL has been re- leased.

The ambition to reproduce normal knee kinematics after the implantation of a TKA has been questioned [16].

Others consider the reproduction of normal kinematic patterns the best option to preserve stability and range of motion [43].As most modern knee prostheses are surface replacements that mimic the anatomical form of the human knee, this seems a logical assumption to us. Normal knee kinematics under loaded conditions (deep knee bend) have recently been studied by dynamic MRI [21], biplanar image-matching of X-rays [2], and fluoroscopy [25, 31]. It has been shown that the posterior part of the medial femoral condyle has a single radius of curvature, acting like a ball-in-socket up from 20° to 110° of flexion, allowing the lateral condyle to pivot around it [21, 24].

This mechanism positions the lateral femorotibial contact point in deep flexion extremely posterior on the tibia. Asano et al. [2] used a biplanar image-matching technique to describe the kinematic behavior of the normal human knee. They found a medial center of rotation in the horizontal plane as the knee moved from full extension to 120° of flexion. The mean axial rotation of the femur relative to the tibia was 22° at a flexion angle of 90°

and 23.8° at a flexion angle of 120°. The medial contact point moved backward by 6.9 mm and the lateral contact point moved backward by 27.4 mm at 120° of flexion. The differences between these fluoroscopic findings and the dynamic MRI data with respect to the kinematics on the medial side of the knee can be attributed to the different description of the kinematic characteristics of the knee.

Depending upon the use of the tibiofemoral contact area or the center of rotation in describing the movement of Chapter 21 · Cruciate Deficiency in the Replaced Knee – J. Victor

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present study use the tibiofemoral contact area.

Mahfouz et al. [31] used fluoroscopy to study and to compare normal knee kinematics with kinematics of the anterior cruciate-deficient knee during a loaded deep knee bend. All ten normal knees displayed femoral roll- back as the knee moved into flexion.At 120° of flexion the mean posterior translation of the medial condyle was 1.9 mm; the mean posterior translation of the lateral condyle was 21 mm; and the mean axial rotation of the femur relative to the tibia was 23.7°. In the anterior cruciate-deficient knee,reduced magnitudes of anteroposterior translation and increased variability in kinematic patterns were observed (reversed axial rotation between 30° and 45° of flexion).

These reported kinematic patterns are difficult to reproduce in patients with TKA. Several authors compared the kinematic behavior of CR implants with those of PS implants. These results are summarized in ⊡ Table 21-1.

Udomkiat et al. [44] found a mean value of femoral roll- forward during flexion on the medial side of 2.7 mm and

medial side and -1.3 mm on the lateral side. Dennis et al.

[15] compared the findings of CR and PS implants from their multicenter data. They reported better lateral roll- back for the PS group as compared with the CR group.

Forward sliding of the medial femoral condyle during flexion was present in 50% of the CR patients and in 70%

of the PS patients. Bertin et al. [8] claimed to have found better kinematic behavior for a CR implant.Of 20 subjects who had undergone a CR NexGen TKA, 13 experienced femoral roll-back on the medial side and 19 on the lateral side. The mean values were -3.1 on the medial side and -3.9 on the lateral side. Although forward sliding of the medial femoral condyle during flexion was far less fre- quently present than in previous reports [5, 6, 16, 20, 22, 44],the absolute value on the lateral side of -3.1 mm is still far from the normal mean value ranging from 21 to 27 mm [2, 31]. Despite the differences in the implants that were used, a clear trend appears from these papers.Axial rotation is less pronounced in the replaced knee than in the human knee, forward sliding of the medial femoral

⊡ Table 21-1.The author’s data compared with a summary of reported kinematics in papers that describe normal human knees or compare posterior-stabilized versus cruciate-retaining total knee prostheses. The first two papers describe the normal knee and serve as a reference baseline.

The other papers compare CR with PS. The data all refer to a loaded deep knee bend

No of Knee Range Medial Lateral Axial % Para- % Para Maximum Method of

knees type of femoral femoral rotation doxical doxical flexion measuring tested motion roll-back roll-back motion, motion, maximum

medial lateral flexion

Asano 6 Normal 0°-120° -6.9 -27.4 23.8°

et al [2]

Mahfouz 10 Normal 0°-120° -1.9 -21 23.7°

et al [31]

Udomkiat 10 CR Apollo 0°-90° 2.7 -2.2 not not not 119 Clinical NWB

et al [43] listed listed listed

Udomkiat 10 PS Apollo 0°-90° 0 -1.3 not not not 118 Clinical NWB

Dennis 20 CR Multi- 0°-90° -0.1 -1.7 -0.1° 50 40 103 Fluoroscopy WB

et al [15] center

Dennis 20 PS Multi- 0°-90° 0.9 -7.1 10.4° 70 0 113 Fluoroscopy WB

et al [15] center

Banks 6 CR AMK 0°-90° 0.4 -5.7 9.6° not not 107 Fluoroscopy WB

et al [5] listed listed

Banks 5 PS 0°-90° 0.2 -1.5 4.9° not not 110 Fluoroscopy WB

et al [5] Osteonics listed listed

Haas 5 CR LCS 0°-90° 0.1 -2.4 -1.4° not not not

Haas 5 PS LCS 0°-90° -2 -5.9 5.3° not not not

Victor 8 CR 0°-115° -2.7 -11.9 6.3° not not 114 Fluoroscopy WB

et al [44] Genesis II listed listed

Victor 7 PS 0°-115° -11.9 -15.5 5.1° not not 117 Fluoroscopy WB

et al [44] Genesis II listed listed

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condyle during flexion on the medial side is present for all CR types of knees, and lateral femoral roll-back is better for the PS than for the CR devices. Maximum flexion tends to be better in the PS than in the CR groups.

In a study performed by the author [45],the difference in maximum flexion during lunge between CR and PS patients was not significant, although the gain in flexion relative to the preoperative status was greater in the PS (14°) than in the CR (5°) group. The better flexion in the PS group correlates with earlier findings [15, 42]. The explanation for this better flexion probably lies in the greater posterior translation of the femur on the tibia [6].

The PS group displayed on average a more posterior contact area in flexion than the CR group (⊡ Figs. 21-4 and 21-5). This posterior position of the femoral component clears the back of the knee and helps to avoid impingement between polyethylene and the posterior cortex of the femur (⊡ Fig. 21-6). This phenomenon is synergistic with the posterior condylar offset as described by Belle- mans et al. as a determinant for maximum flexion [7].

The forward sliding of the medial femoral condyle in CR knees during flexion can be explained as follows: The initial force vector of the patellar tendon keeps the tibia anterior (‘the quadriceps active test’) [12], but with in- creasing flexion, the direction of pull switches to a more vertical position.The gastrocnemius and hamstring mus- cles now exert a posterior pull on the tibia,leading to posterior tibial subluxation in case of an insufficient PCL.

This phenomenon can be partially compensated in PS

knees, as the cam-post mechanism acts as a buttress against posterior subluxation of the tibia relative to the femur (see Fig. 21-5;⊡ Fig. 21-7).

The fact that it still partially occurs with PS knees can be explained by the absence of the stabilizing function of the anterior cruciate ligament. In early flexion, the patellar tendon force vector can translate the tibia anteriorly (see Fig. 21-2). This positions the femur relatively posterior with respect to the tibia.Reduction of this position to the midline causes anterior sliding of the femur on the tibia, which is stopped eventually when the cam-post Chapter 21 · Cruciate Deficiency in the Replaced Knee – J. Victor

⊡ Fig. 21-4. Kinematic behavior of the replaced knee, comparing a cruciate-retaining (CR) against a posterior-stabilized (PS) knee in deep knee bend. The colored lines repre- sent the tibiofemoral contact positions at different angles of flexion.

The contact lines are normalized for a size-five tibial component. The PS knees display a more posterior contact position than the CR knees

⊡ Fig. 21-5.Kinematic comparison of cruciate-retaining (CR) and posterior-stabilized (PS) knee between 50°

and 90° of flexion during stair-step- ping. The forward sliding of the medial condyle in the CR group is ap- parent. The gray dots represent the centers of rotation. In the PS knees they tend to cluster on the medial side, whereas the CR group shows a more scattered pattern

⊡ Fig. 21-6. Anterior sliding of the femur during flexion causes im- pingement between the polyethylene and the posterior cortex of the femur. This mechanism can limit maximum flexion

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mechanism engages. This is supported by the work of Komistek et al. [27]. They compared the in vivo kinematics of an ACL-sparing knee prosthesis with those of a clas- sical PS (ACL-sacrificing) prosthesis and related a posterior tibiofemoral contact position near full extension to an ACL that was ‘not functioning properly’ or absent (PS group). Further confirmation of this hypothesis is found in papers dealing with kinematics following TKA implantation [16, 40], where the authors reported the high- est posterior translation in ACL-sparing total knee de- signs.

Conclusion

Abnormal kinematics and sagittal instability have been reported after implantation of a total knee prosthesis.

These abnormalities are related to deficient ACL and PCL function. Surface congruity and cam-post mechanisms can partially compensate for the sacrificed ligaments if the „flexion gap“ is accurately sized.

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⊡ Fig. 21-7. Fluoroscopic image of two different knees. The total knee arthroplasty on the left (cruciate-retaining) has significantly more anteri- or sliding of the femur with respect to the tibia during flexion than the knee on the right (posterior-stabilized). Flexion is limited in the unstable cruciate-retaining knee because impingement between the posterior polyethylene and the posterior cortex occurs

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