Pain as symptom of threaded acetabular cup loosening

The loosening of acetabular component has been classically described as a more frequent complication than the loosening of femoral component, specially in cemented prostheses. Such description induced the popularization of the usage of non-cemented acetabular components of different designs and materials; at the same time, it became more difficultto distinguish patients with loosened components from those with pain induced by other causes .


INTRODUCTION
The loosening of acetabular component has been classically described as a more frequent complication than the loosening of femoral component, specially in cemented prostheses (7) .Such description induced the popularization of the usage of non-cemented acetabular components of different designs and materials; at the same time, it became more difficultto distinguish patients with loosened components from those with pain induced by other causes (2) .
Results obtained with threaded non-cemented acetabular components was disappointing in many series (4,10,13,15) .In Brazil, the primary total non-cemented hip prosthesis of the CO-10 type (Baumer ® ), which presents an acetabular component of this type, was widely employed during the last decade and their long-and middle-term results are not yet known.In a clinical and radiographic survey of this type of prosthesis carried out at our department, we observed a high rate of acetabular loosening through comparison of serial radiographic images.The present study aims to evaluate and compare the presence and characteristics of pain in patients presenting a fixed acetabular component with those found in patients with radiographically loose.acetabulatcomponent.

MATERIALS AND METHODS
Between November 1989 and December 1995 our service performed 65 total primary non-cemented hip replacements of the CO-10 type (Baumer ® ).
We completed the clinic and radiographic protocol for 37 arthroplasties with 5-to 10-year follow-up.Arthroplasty patients were excluded in the following cases: postoperative infection, no follow-up, incomplete radiological file, and presence of radiographic signs of femoral loosening.The average age of patients was 49.3 years with an average follow-up of 94.43 months.
The radiographic evaluation of the acetabular component measured the acetabulum slope and position, these measurements being related to the teardrop.All radiographic measurements were made on a immediate postoperative radiography and on another one with at least 5 years of follow-up; the values obtained were compared for each patient.Correction for the magnification of the radiographies was required.A loose component was defined as that for which measurements, together with observations of the induced bone reaction, indicated component migration (4,8,11) with acetabular component displacement higher than 10% of the value found in the initial radiography.
Pain was clinically evaluated according to the system suggested by Johnston and others (11) , pain being graded as: -Absent: no pain -Mild: light and occasional pain not affecting activities or work -Moderate: pain that made the patient change habits or give up some activities; the patient, however, is still active.-Severe: strong pain causing important limitation Besides gradation, the pain occurrence was checked as: -No pain -Pain at first steps that soon disappears -Pain starting after more than 30 minutes of walking -Pain during the whole walk -Pain all the time Graph 1-Results in percentage of total number of CO-10 acetabula Table 1 -Total #of prostheses CO-10

RESULTS
Out of the 37 cases evaluated, 23 replacements were fixed and 14 showed signs of acetabular loosening.(Tableand Graph 1).
Among the patients presenting the fixed acetabular component, absence of pain was found in 14 patients, mild and moderate pain was found in 6 and 3 patients, respectively, and no patient had severe pain.
As to the occurrence of pain in these patients, 14 had no pain, 3 had pain at the first steps while walking, 3 after more than 30 minutes of walking while no patient had pain during the whole walk and 3 had pain all the time (Table and Graph 2).
Among the patients presenting radiologically loose acetabular component, 4 had no pain while 4, 5 and 3 patients had mild, moderate and severe pain, respectively.
Among the patients who presented a loose acetabular component in the radiographic examination, 4 had no pain in the concerned hip, 4 had pain at the first steps, no patient developed pain after walking more than 30 minutes, 3 had pain during the whole walk, and 3 had pain all the time (Table and Graphic 3) .
Tables and Graphs 4 and 5 show the comparison of pain intensity and occurrence between the two groups.

DISCUSSION
It is assumed that clinically the loose acetabular component causes localized pain in the inguinal area or thigh when walking with load, being worse at the first steps (7) , or pain in the area of the gluteus (2) .
Several authors have reported high rates of loose acetabular components of the threaded non-cemented type (4,10,13,15) , a finding confirmed in our study (38%), while others have shown good results (3,6,9) .Pain is also evaluated through different scores for the study of results following hip replacement procedures (8,13) and is generally shown in a general context.Some studies report rates of 86% in patients with no pain following non-cemented hip replacement (5) , while others show rates of 36% and 60% in patients with pain following this procedure (1,12) .
In 1991, Pupparo and Engh (14) published a study where they verified loosening of 16 units (28.75%) out of a total of 56 threaded acetabula after two to four years of followup.Of the 16, 11 patients (68.75%) had pain and 5 patients (31.25%) had no pain.
Our survey showed a high number of patients with radiologically loose acetabular component.

Comparison between fixed and loose acetabulum
The comparison of the characteristics of grade and occurrence of pain among the patients with the loose acetabular component showed that we cannot trust the pain symptom as an indication of loose acetabular component of the threaded prosthesis of the CO-10 type.
Pain was absent or discrete in 65% of the patients with loose acetabulum.As it would be expected, this percentage was higher for the patients with fixed acetabulum.
If the degree of pain was considered in the diagnostics of these patients, 65% would have been diagnosed as not having a loose acetabulum, since they did not feel pain or had it light and occasional without changing their activities.Weshould also tak into consideration that 35% of the patients with loose acetabulum complained of severe pain, but 13% of the patients with fixed acetabulum also had this typeof complaint.
The analysis of pain occurrence showed that 58% of the patients with loose acetabulum had no pain or had it only during the first steps and therefore, this symptom cannot be considered characteristic of the loosening.Although 42% of this group the pain occurred during the whole walk or all the time, 13% in the group of patients of fixed acetabulum had the same complaint.
The acetabulum loosening is accompanied by loss of the acetabular osseous case, and the larger the loss th more difficult becomes the surgical revision.Every time,it becomes more important the decision of the orthopedist to do a precocious revision of the loose acetabula to avoid an increased osseous destruction.This decision is of greater responsibility when a major surgery is indicated for a patient who has practically no pain; however, our results show that this should be the preferred choice.

CONCLUSION
On the face of the data presented, the authors conclude that, in the total hip non-cemented arthroplasty of the CO-10 type, the pain, alone, is not a symptom of a loose acetabular component and its presence also should not be considered a constant and essential element for this diagnostics.

Table 2 -
Fixed Acetabulum Graph 3 -Results, in percentage, of the loose acetabulum

Table 5 -
Ocurrence of the pain -Comparison between fixed and loose acetabulum

Table 4 -
Intensity of the pain -