Abstract
Instability is an increasingly common cause and symptom of failure of Total Knee Arthroplasty (TKA). Patients seek ‘Functional Stability’, which is the sum of both a balanced joint and, if necessary, mechanical constraint. The objective of this paper is to classify the different types of TKA instability and their causes. Based on this classification, the authors give methodical recommendations for instability management.
Instability classification
Instability in revision TKA can be classified into 3 types based on the management of bone loss and ligamentous deficiency which directs the level of constraint required to achieve functional stability.
Type 1
Bone deficiency: Revision with restoration of joint line and rebuilding the bony anatomy results in a balanced joint. No increased constraint is needed.
Type 2
Ligament and soft tissue deficiency: Requires increased constraint to overcome instability
Type 3
Composite (Total) deficiency: (combined Type 1 and 2).
The multiple causes of instability are outlined for each Instability type along with an algorithm for restoring the joint line and adding titrated constraint to restore functional stability.
KEYWORDS: Balance, Stability, Constraint, Joint line
Introduction
Constraint is not balance
Knee joint balance, constraint and stability are all different parameters, and each must be understood to achieve a successful, functional revision knee replacement. Whilst in primary Total Knee Arthroplasty (TKA) bone loss and ligament insufficiency are not common, both are common in revision TKA. We must understand that simply increasing the level of constraint is not a substitute for a balanced revision TKR, even if stability is achieved. A definition of each term is needed to allow the important differences to be appreciated.
Balance is a dynamic process reliant on ligament function under appropriate tension in all planes of movement. This includes medio-lateral balance as well as the balance between the extension and flexion gap. To achieve this, one must first reconstruct the original bony surface and thus the joint line in extension, flexion and all angles in between. Creating a balanced soft tissue envelope around the knee is crucial.
Constraint is the mechanical substitution of ligament function by implant and bearing design. The surgeon must understand that increased constraint might solve the acute problem of instability, but constraint itself adversely affects bearing wear and fixation. So, the long-term survival rates of hinged knees and varus-valgus constrained (VVC) knees, for example, are not as good as those of CR or PS knees. This leads to the general recommendation for revision TKR of ‘minimal constraint and maximum balance’.
However, what a patient wants is Functional Stability, which is the sum of both a balanced joint and, if necessary, mechanical constraint. The objective of this paper is to classify the different types of instability and their causes. Based on this classification, the authors give methodical recommendations for instability management.
Classification
Instability in revision TKA based on management leads to classification of 3 types (Table 1):
-
1)
Bone deficiency: Needs reconstruction of the original joint line to achieve balance and stability
Table 1.
Knee arthroplasty instability classification.
| Type 1 - Bone deficiency |
| a) Primarily malpositioned, but well fixed |
| b) Secondary malpositioned due to subsidence |
| c) Combination of a and b |
| Type 2 - Ligament and soft tissue deficiency |
| a) Instability caused by ligament deficiency |
| b) Instability caused by neuromuscular disease |
| c) Instability in conjunction with extensor mechanism deficiency |
| Type 3 – Composite (Total) deficiency (combined Type 1 and 2) |
In Type 1 Instability, the aim of treatment is joint line restoration by reconstructing bone defects. In this type the collateral ligaments are intact and by reconstructing the bony surface/anatomy of the femur the joint line is restored and balance is automatically achieved1,2 Failing to reconstruct the joint line leads to clinical problems such as instability, anterior knee pain and limited range of motion.3 Additionally, reconstructing a stable tibial platform is required to achieve appropriate ligament tension and balance.4
-
2)
Ligament deficiency: Needs implant constraint to achieve stability
Titrated mechanical substitution of soft tissues is required for ligament deficiency, from posterior substituting up to a rotating/fixed hinge.5,6 In Type 2 Instability implant position is not the main problem, and although the joint line is correct, the ligaments are insufficient. Depending on the amount and plane of instability, different constraint can be used to achieve stability.
-
3)
Composite (total) deficiency: Needs both joint line reconstruction and implant constraint to achieve stability.
In Type 3 Instability, both bony and ligamentous components are compromised. Firstly, restoration of the joint line by reconstructing bone defects is required, and then stability is achieved by the introduction of constraint. The amount of constraint is again based on the ligamentous situation. Composite deficiency is often seen in extension-flexion gap mismatch. An extreme example of composite (total) instability is when a distal femoral/proximal tibial endoprosthesis is needed and relies on soft tissue envelope tension and mechanical constraint to achieve functional stability. In this specific situation, the joint line is estimated functionally from the patellar articulation, although this requires surgical judgement when severe patella baja or alta are encountered.
Causes and management
TYPE 1 = bone deficiency instability: bone reconstruction to stabilise
Type 1 instability results from:
-
a)
Implant malposition. This problem occurs during surgery and patients are often symptomatic when mobilised and is therefore classified as an early instability7. In this Type 1a instability the implant itself is well fixed. It can be solved by correcting bone resection errors and by placing the revision components in the correct position, remembering that malposition can occur in a single, in two or in all three planes (extension, flexion and rotation). While a varus/valgus malposition is often obvious in the AP view (Fig. 1), an incorrect tibial slope, AP offset, and femoral flexion/extension can be detected in the lateral view. A rotational error of either femur or tibia is often more difficult to detect and CT scans can be helpful.
-
b)
Implant subsidence and bone loss secondary to component loosening and osteolysis requires reconstruction of bone defects. This problem occurs after an initially well-functioning implant and is therefore a late instability8. In this Type 1b instability, ligament balance was originally achieved during primary surgery. Therefore, the knee was well functioning for a long time until loosening began. After subsidence, the originally balanced ligaments become unstable (Fig. 2). However, it is not the ligament itself but the new, altered implant position that produces instability.
-
c)
Combined malpositioning and bone loss, Type 1c, is a combination of Type 1a & b. The Implant was primarily malpositioned and an additional loosening/subsidence causes functional deterioration. Again, in Type 1c the ligaments themselves are not compromised.
Fig. 1.
Typical example of Type 1a instability. Primary implant malpositioning producing medial gapping. Solution is repositioning of the implant. No additional constraint is required.
Fig. 2.
Typical example for Type 1b instability. Tibial subsidence with bone loss is leading to instability. The medial collateral is intact, therefore new implant positioning and fixation is key, no additional constraint required.
Type 1 Management: restoring the Joint Line
The aim in Type 1 instability is rebuilding the original anatomy of femoral and tibia surfaces. The most important factor is the Joint Line, defined as the articulating surface between the femoral component and the insert in extension, flexion and all points in between. In Type 1 instability, reconstructing the joint line is one of the most important factors in achieving normal ligament balancing and normal knee kinematics.9,10 If this is not achieved consequences result, starting with secondary patella baja, anterior knee pain and midrange instability. Symptomatic distal joint line elevation has previously been described and elevation by 8 mm is significant,10 however, more recent papers reduced this value to less than 4 mm.11,12 To restore the joint line, the majority of revision knees require distal and posterior femoral augments.
The position of the tibial component does not affect the joint line directly,13,14 but a stable aligned tibia is the platform from which the joint line articulates. As the position of the tibia affects the joint stability in extension and flexion equally its absolute height is not critical compared to the femoral position. As the workflow in TKA revision is always tibia first, the position of the femoral component is very much affected by the position of the tibia base plate. Therefore, it is crucial to reconstruct varus/valgus position, the slope and height.
3 different aspects of joint line are recognised, extension, flexion and femoral rotation, all of which need to be addressed. In revision TKR, as in primary TKR, re-establishing the natural joint line position will diminish the need for constrained components by optimising ligament balance and hence stability throughout the range of movement. However, due to bone defects the joint line is more difficult to find. Additionally, it has to be mentioned that the flexion gap size is often larger due to larger bone defects of the posterior femur. This is leading frequently to a flexion-extension mismatch (Fig. 3).
Fig. 3.
Flexion-Extension mismatch.
The Joint Line in Extension - The distal articulation of the femoral component effects stability in extension. Proximal migration of the femoral component due to bone loss always requires implant distalisation to restore the joint line. The extension joint line can be estimated by several methods:
-
1)
Distance to medial epicondyle. Depending on the reference and of course on the size of the knee. 25–30 mm distal to medial epicondyle is described as a reproducible reference value11
-
2)
The abductor ratio. In this ratio the width of the femur is divided by the distance of the abductor to the joint line. Different studies have shown a very constant ratio of 0.52.12,15 The ratio of femoral width to the distance from medial epicondyle to joint line is described to be around 0.3. Both parameters overcome the problem of size as they build a relationship between width and distance.
-
3)
Fibula head and Patella. The joint line is estimated by one finger above the fibula head or one finger below the inferior patella pole. These two parameters remain the least reliable and most variable guess of the joint line and cannot be recommended.13 The only exception might be in a situation with a distal femoral replacement, where the position relative to the patella is clinically more relevant than the real anatomical reconstruction of the femoral joint line in extension.
-
4)
Old meniscal scar. This is a beloved and well described reference point16 but is only reliable in a third of cases. In the remainder the ‘pseudo meniscus’ is no more than fibrous ingrowth related to where the previous replacement was implanted.17
-
5)
Accurate primary resection. Provided the revision is performed for reasons other than distal femoral proximalisation, then the original distal femoral resection can be used as a ‘Primary’ point (i.e. thickness of distal femoral component ~9/10 mm) from which the joint line can be estimated i.e. the original primary distal resection is preserved needing only the implant dimension (distal thickness) without augment to restore the extension joint line. However, in the vast majority of revision cases distal augments are required. Removing a well fixed femoral implant generally leads to some bone loss e.g. soft/osteopaenic bone, polyethylene granulomata and PS implants.
The Joint Line in Flexion - The posterior femoral condyles affect stability in flexion. Three different parameters are relevant in restoring the femoral sagittal anatomy, the overall size of the femur, the anterior and posterior offset. All three should be reconstructed to avoid functional changes and instability problems11,18,19 Bellemans et al. described a linear relationship between reduced femoral offset and loss of flexion. An increased size of the femur and/or anterior offset on the other hand can lead to anterior knee pain and limited ROM.20 However, femoral component posterior offset is often dictated by long stem diaphyseal fixation which anteriorises the femoral condyles. This is mitigated by posterior offset options, femoral sizing and posterior augments. However, even if posterior offset is reconstructed, the physiologic flexion of the Zone 1 and 2 in relation to Zone 3 with such constructs is not fully restored. Short cemented stem fixation and metaphyseal sleeve fixation both allow the normal distal femoral morphology to be recreated with a ‘flexed’ position, automatically closing the ‘flexion gap’21 (Fig. 4).
Fig. 4.
In this figure the effect of flexing the femoral component on the osterior femoral offset is demonstrated. While in the left figure, the implant is fixed in the diaphysis, on the right figure it is fixed in the metaphysis. This is leading to a more flexed position of the implant which corresponds to the anatomical situation in most of the femurs. This flexed position is increasing the offset.
Flexion by 5° has been shown to close the flexion gap by 7.8 mm.13,18 Significant extension-flexion asymmetry may require additional constraint with a large post or rotational hinge, if reconstruction options are exhausted.
The Joint Line in Rotation - The joint line in rotation affects medio-lateral balance and stability in flexion, posteriorly affecting the femoral-tibial articulation and, anteriorly, the patello-femoral function. Just as the distal femur must be augmented, so too must the posterior condyles, often asymmetrically, to restore the rotational joint line. An additional factor resulting in patello-femoral instability can be malrotation of the tibia.22 To achieve rotational stability different techniques, exist. Measured resection surgeons take the anatomical landmarks, e.g. epicondylar axis or posterior condyles as reference. In revisions these landmarks are often compromised. Therefore, a balanced gap approach is widely used. Based on a stable and well-aligned tibial platform, the femoral component rotation is set based on the ligament tension. In all type 1 instabilities, the ligaments are sufficient and therefore this technique is reliable but becomes more difficult with ligament compromise.
TYPE 2: Ligament instability: Requires implant support/constraint to achieve stability
Type 2 Instability results from a) ligamentous deficiency, b) extensor deficiency or c) neuromuscular disease.
Type 2a ligamentous instability maybe from either a pre, intra or postoperative ligamentous injury e.g. medial collateral or posterior cruciate injury.
Type 2b is an instability that is combined with an additional extensor mechanism deficiency which produce hyperextension gaits and posterior capsular laxity
Type 2c includes such diseases as Poliomyelitis, Guillian-Barre syndrome, incomplete spinal cord injury etc. which may produce global capsular laxity.
Instability management in Type 2 cases are often determined by the direction of instability (coronal/sagittal/global) and by titrating varying levels of constraint.
Coronal instability
Depending on the quality of the ligaments we can differentiate between laxity, partial deficiency and complete deficiency. While Medial Collateral Ligament (MCL) laxity, as seen in significant valgus deformities, may be managed without constraint in most of patients, partial MCL/LCL deficiency needs implant support with higher amount of Varus/Valgus Constraint.23 In complete MCL deficiency (Fig. 5), increased constraint, such as a rotating hinge, is required to achieve a stable joint.24 Complete LCL deficiency is often accompanied by popliteus and lateral capsule deficiency which again makes maximal constraint necessary. If the LCL only is involved a varus/valgus constraint is mostly sufficient.
Fig. 5.
Two typical example of Type 2a instability with complete ligament insufficiency. A) Is showing a complete MCL insufficiency; B) a complete PCL insufficiency.
Sagittal instability
Isolated PCL deficiency leads to a sagittal instability, requiring higher conformity or a posterior stabilised (PS) bearing (Fig. 5).
Global instability
Combined sagittal and coronal is referred to as global instability. This can often be found for example in Type 2b instability. This severe form of instability always requires a rotating hinge constraint. The amount of hyperextension that the patient might need for stabilizing the knee needs to be assessed before surgery and may determine the specific hinge design that should be chosen.25
Type 2c: instability plus extensor mechanism deficiency (Fig. 6)
Fig. 6.
Typical example for Type 2c instability: Extensor mechanism deficiency in a failed revision.
This is a specific subgroup. Extensor failure should, where possible, be managed with reconstruction.26 Successful reconstruction may allow the use of a varus-valgus-constraint bearing or a rotating hinge. Unsuccessful reconstruction warrants a fixed hinge to prevent a hyperextension gait and improve function. The long-term follow-up of such cases is unknown in revision TKR patients but would possibly be comparable to fixed hinge replacements for proximal tibial tumour prosthesis.27,28
TYPE 3 – Composite (total) instability (a combination of Type 1 & 2)
Type 3 instability results from a mix of pathologies, including capsular distension, implant instability, the multi-operated knee and the need for massive endoprosthesis (Fig. 7). This leads to instability in coronal (medio-lateral), sagittal (antero-posterior), and global planes. A titrated approach ranging from posterior stabilised through to varus-valgus constraint to a rotating hinge is necessary. Different subtypes of this Type 3 instability exist.
-
a)
Capsular distension secondary to, e.g., polyethylene synovitis, metallosis or infection, is managed by removal of worn or infected implants, synovectomy and bone reconstruction. Constraint can be increased as indicated after bony reconstruction.
-
b)
Implant positioning. Zone 3 (diaphyseal) stem placement leading to anteriorisation of femoral condyles and mismatched extension-flexion gaps and requires greater constraint from a VVC post. Zone 2 (metaphyseal) fixation can prevent this problem.
-
c)
Multi-operated knees (post-trauma or replacement/revision) often presents with major bone loss and soft tissue compromise. These cases often present with stiffness and require significant exposure and dissection to explant and reconstruct. In such cases VVC and rotating hinges are the norm.
-
d)
‘Total’ instability resulting from massive bone loss or excessive resection (e.g. periprosthetic fracture) which compromises the capsular and ligament attachments necessitates a rotating hinge as part of a Distal Femoral or Proximal Tibia endoprosthesis. The joint line is commonly referenced from the patella height (Fig. 8). Stability also relies on the soft tissue envelope tension and mechanical hinged constraint (for absent ligaments/capsule) to achieve functional stability. When combined with extensor failure a fixed hinge should be considered.
Fig. 7.
Typical example of Type 3 instability: Massive bone destruction and capsular distension due to tibial loosening and polyethylene wear.
Fig. 8.
Typical example if Type 3d instability: Major femoral bone loss with valgus deformity and posterior lateral ligament insufficiency.
Type 3: Management requires reconstruction and titrated constraint
In this type of instability, we recommend firstly managing the bone defects as per Type 1 Instability. After solid fixation of the tibial component and reconstruction of the femoral joint lines the amount of ligamentous deficiency should be again analysed. Based on the amount of deficiency the amount of constraint is defined as with Type 2 instability. A lower threshold for rotating hinge implants is advised in Type 3 instability.
Conclusion
Instability in revision TKR can be classified into 3 types based on the management of bone loss and ligamentous deficiency which directs the level of constraint required to achieve functional stability. These 3 types are:
Type 1 - bone deficiency
-
a)
Primarily malpositioned, but well fixed
-
b)
Secondary malpositioned due to subsidence
-
c)
Combination of a and b
Type 2 - Ligament and soft tissue deficiency
-
a)
Instability caused by ligament deficiency
-
b)
Instability in conjunction with extensor mechanism deficiency
-
c)
Instability caused by neuromuscular disease
Type 3 – Composite (Total) deficiency (combined Type 1 and 2)
Different reasons for instability in revision TKR exist. Mostly instability is from bony/implant causes and not ligament deficiency (Type 1) and results from bone loss, component malpositioning, and failure to restore the joint line. Revision with restoration of joint line and rebuilding the bony anatomy results in a balanced joint. As the function and structure of the ligaments is not altered in Type 1 instability, minimal constraint is required to achieve functional stability.
True ligament injury, hyperextension and global instability (Type 2) require increased constraint up to a hinged knee replacement if necessary. The constraint dependent on the instability. While medial instability almost always needs a rotating hinge, lateral instability can be treated with VVC constraint, as long as the posterior lateral capsule and the popliteus are still sufficient.
In Composite Type 3, a combination of bony and ligamentous instability is present, and both causes need to be addressed to achieve a stable and functional joint. First, the bone defects need to be reconstructed, and then instability addressed by adequate constraint. Extreme bone loss and ligamentous loss leading to ‘total’ instability results from massive bone loss or bone resection. This requires endoprosthesis reconstruction, appropriate tension in the soft tissue envelope, referencing off the level of the patella where possible and a hinged bearing.
Declaration of competing interest
No Declarations of interest
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