By Henry D. Clarke, MD
Background
The principal effect of posterior cruciate ligament release is a tendency for the flexion gap to open up more than the extension space1,2. This requires subtle differences in bone cuts and soft-tissue balancing in cruciate-retaining versus posterior stabilized total knee arthroplasty.
This review will be particularly helpful to surgeons who use different prostheses in different demographic groups and those who occasionally make an intraoperative conversion from a cruciate-retaining to a posterior stabilized prosthesis. The problems encountered in cruciate-retaining and posterior stabilized knees reflect the effect of posterior cruciate ligament release.
Problems in cruciate-retaining knees include:Limited flexion when the posterior cruciate ligament is too tightParadoxical roll forward or flexion instability if the posterior cruciate ligament is too loose
Limited flexion when the posterior cruciate ligament is too tight
Paradoxical roll forward or flexion instability if the posterior cruciate ligament is too loose
Problems in posterior stabilized knees include:Posterior dislocation of the femur on the tibia (rare)Flexion contracture
Posterior dislocation of the femur on the tibia (rare)
Flexion contracture
Differences in bone cuts in cruciate-retaining and posterior stabilized total knee arthroplasty may help to avoid these detrimental outcomes:
In cruciate-retaining knees:A thinner distal femoral resection helps avoid use of a thicker polyethylene that may make the posterior cruciate ligament too tight.Posterior slope of the tibia helps flexion by opening up the flexion space.
A thinner distal femoral resection helps avoid use of a thicker polyethylene that may make the posterior cruciate ligament too tight.
Posterior slope of the tibia helps flexion by opening up the flexion space.
In posterior stabilized knees:Additional distal femoral resection may be required to allow use of a tibial polyethylene insert that adequately fills the flexion gap without a residual flexion contracture.Avoid excessive posterior slope of the tibial component (no more than 5°) to avoid potential anterior impingement of the tibial post.
Additional distal femoral resection may be required to allow use of a tibial polyethylene insert that adequately fills the flexion gap without a residual flexion contracture.
Avoid excessive posterior slope of the tibial component (no more than 5°) to avoid potential anterior impingement of the tibial post.
Differences in soft-tissue releases may also be considered in cruciate-retaining and posterior stabilized knees:
For the cruciate-retaining knee:If the knee is tight in flexion, posterior cruciate ligament "balancing" or partial release may be considered; however, in knees that undergo this treatment, there is a concern for late failure of the posterior cruciate ligament.An alternative is the use of minus or half-size components that incorporate a reduction in the thickness of the posterior condyle for any given medial-lateral dimension, which relaxes the posterior cruciate ligament.
If the knee is tight in flexion, posterior cruciate ligament "balancing" or partial release may be considered; however, in knees that undergo this treatment, there is a concern for late failure of the posterior cruciate ligament.
An alternative is the use of minus or half-size components that incorporate a reduction in the thickness of the posterior condyle for any given medial-lateral dimension, which relaxes the posterior cruciate ligament.
For the posterior stabilized knee:In the valgus knee, avoid release of both the lateral collateral ligament and popliteus tendon because of the potential risk of dislocation in the flexed, figure-four position.When faced with a small residual intraoperative flexion contracture (<10°) with a posterior stabilized knee, the surgeon may incorporate a posterior capsular release rather than additional distal femoral resection.
In the valgus knee, avoid release of both the lateral collateral ligament and popliteus tendon because of the potential risk of dislocation in the flexed, figure-four position.
When faced with a small residual intraoperative flexion contracture (<10°) with a posterior stabilized knee, the surgeon may incorporate a posterior capsular release rather than additional distal femoral resection.
Prosthetic solutions:
With both cruciate-retaining and posterior stabilized prostheses, the goal is to try to restore the femoral anatomy, including the posterior condylar offset. In a posterior stabilized knee, avoid routine downsizing because this will increase the flexion gap and create the potential for flexion instability. In a cruciate-retaining knee, attention must be given to joint-line position in an effort to maintain the kinematics of the posterior cruciate ligament. An implant with a decreased anterior-posterior dimension for a comparable medial-lateral width (a minus-size femoral component) may help maintain flexion stability with an intact posterior cruciate ligament and allow better flexion.
Conclusions
Release of the posterior cruciate ligament is likely to increase the size of the flexion gap more than that of the extension gap. In posterior stabilized knees, flexion contracture is the most common result and can be eliminated by additional distal femoral resection or posterior capsular release. In a cruciate-retaining knee, tightness in flexion may require posterior cruciate ligament balancing or the use of a minus-size femoral component.
Clinical Relevance
Knowledge of the effect of posterior cruciate ligament release is particularly important for surgeons who usually use cruciate-retaining designs but who occasionally use a posterior stabilized prosthesis.
Management of Extra-Articular Deformities In Primary Total Knee Arthroplasty
By Michael A. Mont, MD, Mike S. McGrath, MD, and Michael G. Zywiel, MD
Extra-articular deformities of the lower extremity, such as bowing or angulation of the femur or tibia, may occur as a result of trauma or various metabolic bone diseases and can substantially alter the mechanical axis. When performing a total knee arthroplasty in a patient who has an extra-articular deformity, additional planning is necessary to avoid complications such as component malpositioning, patellofemoral maltracking, or ligamentous imbalance1-3.
When evaluating an extra-articular deformity, full-length standing radiographs are essential to assess the mechanical axis of the lower extremity1. In addition, the surgeon should determine the magnitude of the angulation and the distance from the knee joint, as both have a substantial impact on the mechanical axis2. Deformities that are closer to the knee joint have a proportionally greater effect on the alignment of the joint. Alignment in the sagittal plane and rotational deformities can add to the complexity2.
Treatment options include standard intra-articular corrections, intra-articular correction with collateral ligament reconstruction, simultaneous osteotomy with total knee arthroplasty, two-stage osteotomy followed by total knee arthroplasty, or the use of a highly constrained prosthesis1. The clinical results associated with simultaneous or two-stage osteotomies followed by total knee arthroplasties are good to excellent, with significantly improved mechanical axis angles4-8. However, this procedure is technically demanding, and there is a risk of nonunion of the osteotomy site4-8. Intra-articular bone cuts have been associated with varying results, ranging from poor to excellent9,10. However, the surgeon needs to be aware that oblique bone cuts to correct the mechanical axis may lead to laxity of the collateral ligament on the side that is associated with a larger resection, and collateral ligament reconstruction may be necessary. At our institution, 71% of knees that had extra-articular deformities were treated with intra-articular bone cuts alone, whereas the remainder required more extensive procedures, such as ligament reconstruction or osteotomy.
Extra-articular bone deformities can create an additional challenge to the performance of a total knee arthroplasty. Various surgical techniques have been utilized to treat these deformities, and further study is necessary to gain a greater understanding of the optimal approach.
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McGrath MS, Suda AJ, Bonutti PM, Zywiel MG, Marker DR, Seyler TM, Mont MA. Techniques for managing anatomic variations in primary total knee arthroplasty. Expert Rev Med Devices.2009;6:75-93.675
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What Is the Evidence for Mobile-Bearing Total Knee Arthroplasty?
By Douglas A. Dennis, MD, Joshua Carothers, MD, and Raymond H. Kim, MD
Background and Premise
The introduction of mobile-bearing total knee arthroplasty was accompanied by several theoretical advantages, including reduced polyethylene wear, reduced fixation stresses, and ultimately, a reduced rate of revision total knee arthroplasty. Clinical comparisons of fixed-bearing versus mobile-bearing total knee arthroplasty have not demonstrated significant differences, although such studies have been limited in number and in duration of follow-up. Available clinical evaluations were reviewed to evaluate these claims and to compare the various types of mobile-bearing total knee prostheses with regard to outcome.
Methods
An extensive database search was completed for the purpose of performing a meta-analysis of studies in which outcomes of mobile-bearing total knee prostheses were reported. Inclusion requirements for manuscript analysis were a minimum 4.5-year duration of follow-up and the reporting of knee scores, motion, loosening rates, complications, and survivorship. Both retrospective and prospective trials were included. A total of 1855 reports were initially identified, and eighteen manuscripts (average duration of follow-up, 8.6 years) met the criteria for analysis. Data were subdivided on the basis of the type of mobile-bearing design and included rotating-platform, meniscal-bearing, and rotation-anteroposterior translation subgroups.
Clinical Results
The fifteen-year survivorship associated with rotating platform designs (96.4%) was greater than that associated with meniscal-bearing implants (86.5%; p < 0.003). Mean component loosening across all mobile-bearing implant subgroups was 0.33%. Bearing instability was uncommon (=1%). Implants placed prior to 1995 exhibited higher rates of bearing instability (1.3% compared with 0.12%; p < 0.002). Prospectively randomized trials comparing fixed-bearing and mobile-bearing total knee prostheses were very limited and demonstrated similar results, although the duration of follow-up was short.
Conclusions
Within the course of the last two decades, excellent clinical results and low revision rates have been obtained with use of mobile-bearing total knee prostheses. Loosening rates and polyethylene wear rates have been very low. The occurrence of bearing instability, an uncommon mode of failure, has lessened as surgeons have gained experience with mobile-bearing total knee arthroplasty.
Clinical Relevance
The clinical results associated with fixed-bearing and mobile-bearing total knee prostheses have been reported to be similar after a decade of follow-up. The initial fears of substantial backside wear and bearing instability with use of a mobile-bearing total knee arthroplasty have not been realized. Prospectively randomized controlled studies with a longer duration of follow-up are required to determine if the survivorship associated with mobile-bearing designs is superior to the survivorship associated with fixed-bearing total knee designs.
Bhan S, Malhotra R, Kiran EK, Shukla S, Bijjawara M. A comparison of fixed-bearing and mobile-bearing total knee arthroplasty at a minimum follow-up of 4.5 years. J Bone Joint Surg Am.2005;87:2290-6.872290
2005
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Buechel FF Sr. Long-term followup after mobile-bearing total knee replacement. Clin Orthop Relat Res.2002;404:40-50.40440
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Callaghan JJ, O'Rourke MR, Iossi MF, Liu SS, Goetz DD, Vittetoe DA, Sullivan PM, Johnston RC. Cemented rotating-platform total knee replacement. A concise follow-up, at a minimum of fifteen years, of a previous report. J Bone Joint Surg Am.2005;87:1995-8.871995
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