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A comparison of bone loss at total knee replacement: posterior stabilized versus cruciate retaining.

Our study compares two well established designs of knee replacement, cruciate retaining versus posterior stabilized. Using 20 saw bone models we were able to demonstrate that more bone stock was lost using the posterior stabilized design. Our concern is that reduced bone stock may have implications both at the time of surgery and impact on the longevity of the knee replacement.

Ruth Wragg MBBS SHO Orthopaedics

Riaz JK Khan FRCS (Tr & Orth) Senior Lecturer

Iswadi TW Damasena MBBS, Orthopaedic Registrar* 

James Wimhurst MChir FRCS (Tr & Orth), Consultant Orthopaedic Surgeon


Institute of Orthopaedics ,Norfolk and Norwich University Hospital,

Colney Lane ,Norwich ,NR 7UY


Institute of Orthopaedics, Norfolk and Norwich University Hospital,

Colney Lane, Norwich ,NR 7UY


Dept of Surgery and Pathology*

University of Western Australia

Perth, WA 6009




Corresponding Author:

Dr Iswadi Damasena

2/25 Hardy Street

South Perth

WA 6151

Tel +61438931080

Fax +61893863015



The posterior stabilized and cruciate retaining are well established designs of knee replacement. Concerns have been raised with regard to the amount of extra bone excised when inserting a posterior stabilized knee replacement.


We compared the amount of sawbone excised in the two different types of prosthesis in two different makes used at our hospital (Genesis 2 and PFC). The Sawbones were weighed before and after preparation for the cruciate retaining prosthesis and then the posterior stabilized prosthesis. The weight of bone loss in each of the designs was measured. Ten sawbones were used for the each of the four groups

Results 1

5% more bone was lost in the Genesis 2 posterior stabilized design compared to the cruciate retaining design (P<0.005). 25% more bone was lost in the PFC posterior stabilized design compared to the cruciate retaining design (P<0.005). Further, 7% more bone was lost using the PFC posterior stabilized model rather than the Genesis 2 posterior stabilized model (P<0.001).


We conclude that significantly more bone is excised using the posterior stabilized design, that this may have consequences at the time of surgery, and impact on the longevity of the knee replacement, such that in the absence of specific indications for a posterior stabilized knee, consideration should be given to a cruciate retaining prosthesis.


Knee arthroplasty; posterior stabilized; cruciate retaining; bone loss

J.Orthopaedics 2011;9(1)e4



CR – cruciate retaining

PCL – posterior cruciate ligament

PFC – press fit condylar

PS – posterior stabilized

TKR – total knee replacement


Introduction :

There are two major classes of total knee replacement (TKR): the posterior stabilized (PS) and the cruciate retaining (CR). Studies have demonstrated little advantage of one over the other in terms of the final functional result. Concerns have been expressed with regards to the amount of bone excised during femoral preparation for the PS knee replacement compared to the CR prosthesis. More recent designs of PS knees have attempted to address this issue.

The purpose of this study was to quantify the amount of bone lost from the femur in the two designs (PS vs CR), and to compare two commonly used makes of prosthesis (Genesis Two, Smith and Nephew, (Cambridge UK) and PFC, Depuy (Leeds, UK))


1. Subjects of Study

Twenty femoral “sawbones” (Sawbones, Malmo, Sweden) were used for the experiment. Ten were allocated to the Genesis 2 group and ten were allocated to the PFC group.

Each sawbone was weighed using digital scales (A&D EK series, Progen Scientific (South Yorkshire, UK)). The weight was recorded in grams to the nearest 0.1gram. The sawbone was then prepared for a CR knee replacement. In the Genesis 2 group, the femur was correctly sized to a size 6 and in the PFC group to a size 4. The cuts were made with the appropriate jigs (with 5 degrees of valgus) by one author (RK). All debris was removed, and the sawbones were then weighed again.

The additional cuts were made for a posterior stabilized knee replacement using the appropriate blocks, and the bones were re-weighed.

The Difference in weight between the first and second reading is the weight of sawbone removed for a cruciate retaining knee replacement. The difference between the first and third reading is the weight of sawbone removed for the posterior stabilized knee replacement.

Data was collected on an Excel spreadsheet . The data was not normally distributed and therefore statistical analysis was performed using a Wilcox Rank Test for the paired data and a Mann Whitney U test for the unpaired data.


Results The amount of bone resected for each group is shown in Table 1. During preparation of the femur for a PFC PS knee replacement, 25% more sawbone was resected compared to the cruciate retaining knee replacement (13.25g vs 10.60 g, p<0.005). Similarly in the Genesis 2 PS knee replacement, 15% more sawbone was resected compared to the cruciate retaining knee replacement (mean 12.37g vs 10.67g, p<0.005). Further, 7% more bone was lost using the PFC PS design compared with the Genesis 2 PS design (13.25g vs 12.37g, p<0.0001). There was no significant difference in the amount of bone resected by the PFC or Genesis 2 CR knees (P=0.315)



Total Knee Replacement is a well established treatment for arthritis of the knee. The success of the two main designs discussed here (posterior stabilized and the cruciate retaining knee replacement) have been well documented. The PS TKR was designed to confer greater stability to the TKR, however it is increasingly being used routinely.

The on-going debate regarding the relative merits of the PS design over the CR design continues unresolved. The relative indications for the PS design include deficiency of the posterior cruciate ligament (PCL), and mild knee instability1-2. Theoretical advantages include better proprioception and easier balancing of the knee3. However, studies comparing the two designs report equivocal results. Wang et al4 studied clinical outcome (based on knee and functional scores) and radiological changes in the two groups at 2-5 years. They found no significant difference between the groups. Maruyama et al5 studied 20 patients with bilateral knee replacements. One knee in each patient was replaced with the CR design, and the other with a PS design. No difference was found between the knee scores of the two designs. Other studies have reported similar results6-8

Straw et al6 performed a study looking at PCL retention, excision and substitution. Their patients were randomized to either having a CR TKR, a PS TKR or a CR TKR with excision of the PCL. They reported no significant difference in the functional scores or range of movement obtained by any of the three methods. In patients they had randomised to have retention of the PCL who they subsequently found to have a ligament that was too tight, they performed a release of the ligament to a suitable tension. This subgroup did have a significantly worse outcome.

A study by Swanik et al9 assessed proprioception and balance in the two designs, but found no difference between the two groups. Misra et al10 performed a similar study comparing the CR prosthesis with the PCL left intact and the PCL having been sacrificed. Again, no significant difference in outcome was found

. In one of the few studies to identify a difference in outcome between the CR and PS knees, Andriacchi and Galante11 assessed gait analysis. They reported an improved range of motion and a more normal gait pattern (on stair climbing) in the CR group. Interestingly Ishii et al12 found no difference in gait during the swing phase, but did note an increase in abduction and adduction as well as proximal and distal translation rotation in the posterior stabilized group, implying less stability; however the difference was not statistically significant.

A study by Hass et al13 compared bone loss between five different types of PS implant devices. They noted significant differences in bone loss between different implants and suggested that this may affect the in vivo kinematics of the knee, predispose to intercondylar fracture, leave less bone at time of revision, and ultimately affect the longevity of total knee arthroplasty. Unlike ours, this study did not directly compare the CR prostheses with the PS knee, however it must be suspected that resection of more bone stock such as in PS knees may predispose to complications that may otherwise be avoided in CR knee arthroplasty.

Our study suggests that significantly more bone is resected at surgery for the PS knee replacement compared to that of the CR design, and that there is a significant difference between the specific designs from different companies. Although beyond the scope of this study, two other factors should be considered. First, unlike the CR design, the inset box shape of the PS design TKRs makes it impossible to pass an osteotome or saw blade between the prosthesis and bone interface. Thus, on removal, the PS prosthesis tends to remove more host bone. Second, the PS design does not permit the passage of a nail in the event of a distal femoral periprosthetic fracture.

The study may be criticised for using sawbones rather than real bone. However, an in vivo study presents significant technical problems: accurate measurement of the amount of bone resected would require collection of bone dust as well as resected fragments, making it virtually impossible. Furthermore, the presence of other matter (blood, soft tissues, saline wash etc) would result in marked inaccuracies. The advantage of the method used is the highly accurate assessment of the weight of material resected, presented as a proportion of the overall weight of sawbone. A second criticism may be that sawbones are of a homogenous density unlike real bone. However, since the extra bone resected to accommodate a PS knee is mainly cortical, measurements using sawbones are likely to represent a conservative assessment of the true weight of bone removed in vivo. Finally, no studies have established the clinical relevance of removing more or less bone at primary arthroplasty. Nevertheless, it is intuitive that bone preserving techniques are better, and innate in orthopaedic surgeons to remove no more than is required.

The PS TKR was designed to confer greater coronal plane stability, however it is increasingly being used routinely. In terms of bone loss it may be more appropriate to choose a CR design at primary surgery in an uncomplicated knee. If however a PS design is indicated, consideration should be given to selecting a model which resects less bone.

Table 1: Comparisons of the mean amount of bone resected in the different designs and models of knee replacements.


Cruciate Retaining

(Mean weight in grams ± SD)

Posterior Stabilized

(Mean weight in grams ± SD)

P value *




Bone removed


143.46 ± 3.84

132.86 ± 3.19

10.60 ± 1.53


143.46 ± 3.84

130.21 ± 3.24

13.25 ± 1.64





Genesis 2



Bone removed


141.26 ± 4.84

130.59 ± 4.57

10.67 ± 0.84


141.26 ± 4.84

128.89 ± 4.49

12.37 ± 0.95





P value **



* p value calculated using Wilcox Rank Test

** P value calculated using Mann Whitney U test


Figure 1: Sawbone following the cuts being made for the cruciate retaining design

 (PFC on the left, and Genesis 2 on the right).

Figure 2: Sawbones following the cuts being made for the posterior stabilized design

 (PFC on the left, and Genesis 2 on the right).

Author contributions

Ruth Wragg

Involved in experimental design, preparation of manuscript, formatting, editing and carried out statistical analysis

Riaz JK Khan FRCS 

Project supervisor, assisted in experimental design, preparation of sawbones, editing, formatting,

 technical support, and conceptual advice. 

Iswadi TW Damasena  

Involved in preparation of manuscript, formatting, editing, and submission.

James Wimhurst


Project supervisor along with R.Khan.




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  • This is a peer reviewed paper 

    Please cite as :A comparison of bone loss at total knee replacement: posterior stabilized versus cruciate retaining.

    J.Orthopaedics 2012;9(1)e4




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