Outcomes after total hip replacements

Even the operation of the century has a dark side. The image is CC by Holley And Chris Melton.
Even the operation of the century has a dark side. The image is CC by Holley And Chris Melton.

While total hip replacements (THR), also known as total hip arthroplasties, are hugely successful there still are, and will always be, poor outcomes. This post is an excerpt from my thesis where I tried to summarize the dark side of THR.

The success of a THR is measured by 4 main outcome categories: (1) patient survival, (2) implant survival, (3) other adverse events, and (4) patient-reported outcome measures (PROM). Although technical outcomes such as component positioning, soft tissue preservation or leg length restoration can be viewed as a type of outcome, they all aim at addressing issues within the 4 main outcomes.

Patient survival

The patient surviving the surgery is the most fundamental premise for any kind of surgery. The mortality associated with the THR procedures occurs soon after the surgery; effects related to bleeding, pulmonary embolism, and other per-operative causes quickly decline during the first months [1]. The 90-day mortality rate is the most common mortality measure [2], although some studies report in-hospital mortality [3] or 30-day mortality [4].

Prior to the aseptic technique introduced in late 19th century, mortality rates of 50% were common [5]. Even today there seems to be improvement in the mortality rates [6]–[8], and the current 90-day mortality in Sweden is less than 1% [9], similar to rates in other countries [1], [3], [6], [7], [10]–[14].

Implant Survival

Implant survival relates to the survival of the implanted material. There are two terms frequently used for estimating implant survival, re-operations and revisions:

  • Re-operation is a wider definition whereby any further surgery affecting the operated hip is included.
  • Revision is a subgroup of re-operations including only the surgeries where the implant is exchanged.

The Swedish Hip Arthroplasty Register (SHAR) has from its start in 1979 used re-operations as a measurement [15]. Re-operations and revisions can be further subdivided into the underlying cause:

  • Infection: Often referred to as periprosthetic joint infection, it occurs most frequently within the first year of surgery [16], [17] with an estimated 2 year cumulative incidence of 1% [18].
  • Fracture: Periprostethic fractures occur both early and late after THR. Early re-operations within 6 months are frequently technical complications and are associated with cementless stems [19]. The average time to fracture is 7 years, and the estimated cumulative incidence about 1% for 5 years, and 3-4% at 10 years [20], [21].
  • Instability: A THR dislocates more easily than the innate hip joint. The incidence of dislocation is difficult to measure and ranges in international studies between 1 and 5% [22]–[25]. Between 2009 and 2012, 0.3% in Sweden were re-operated within 2 years due to dislocation [26, s 52]. Most dislocations occur early on, 50%-70% occur within the first 3 postoperative months [27], [28], although late dislocations (> 5 years) more often require re-operation [29].
  • Loosening/lysis: Loosening occurrs whenthe bone retracts from the implant with a radiolucent line visible on x-rays. If the process is localized and the implant is clearly not loose, it is referred to as osteolysis [30]. Common inflammatory pathophysiology for the processes has been hypothesized [31], [32]. The cumulative incidence for loosening/osteolysis is 0.2% at 2 years, 0.8% at 5 years, and 2% at 10 years (study I).
  • Other: There are multiple other causes such as technical failure or implant fracture for re-operations that amount to a cumulative 10 year incidence of less that 3% (study I).

The 10 year re-operation rate is currently at an all-time low, less than 5% for patients operated due to primary osteoarthritis [9]. This coincides with the new having increasing difficulty in improving on the existing designs [33], currently 3 stems constitute 99% of all implanted cemented stems in Sweden [26, s 13].

Adverse events

Death and re-operations are only a subset of adverse events after THR. Other events such as cardiovascular, thromboembolic events can be equally important, while minor events such as urinary tract infection may be quality of care indicators. Adverse events in medical practice are fairly common, expected around 3-15% [34]–[38], and perhaps even more common in orthopaedics [39]–[41]. There is currently no validated registry-based tool for the Swedish population, although readmissions combined with ICD-codes are reported on a yearly basis in Sweden [9]. Our research group is currently working on validating a new tool for measuring adverse events that we hope will allow us to study this closer in the future.

Patient-reported outcome

The THR indication is primarily to improve quality of life, and is very successful at this [42]. Charnley reported in his landmark paper on long-term results in 1972 that the majority of patients had excellent results regarding pain and walking ability [43]. As the indications have widened and patient demographics changed, the importance of patient-reported outcome measures (PROMs) have reentered center stage [44].

Furthermore, with the improvement in implant survival and low re-operation,PROMs are becoming increasingly important for evaluating THR [45]. Although hip resurfacing has fallen out of favor, many of its main proponents argued for it because of better PROMs [46]–[48].

In orthopaedics there are two main types of PROMs: generic measures and disease specific measures. Generic measures usually span different health domains, and can be used irrespective of the particular disease being studied; this has the advantage of allowing comparison between different diseases [49]. Generic measures are frequently also referred to as health related quality of life (HRQoL). There are 2 main HRQoL instruments used in orthopaedics:

  • Euroqol 5 dimensions (EQ-5D): The EQ-5D™ was developed in the late 80s by the EuroQol group as a standardised, non-disease-specific instrument for describing and evaluating health states[50]. The tool consists of 5 different dimensions and avisual analogue scale (VAS). The dimensions are weighted according to a value set in an index where values of 0 correspond to states equal to death and 1 to perfect health. The 5 dimensions are:
    1. Mobility
    2. Self care
    3. Usual activities
    4. Pain
    5. Depression
  • Short Form 36 (SF-36): The SF-36® has its roots in the 70s but was formalized in the late 80s[51], and is now managed by the Medical Outcomes Trust. The tool consists of 36 items that are aggregated into 8 different domains that in turn can be divided into physical and mental health summaries. The domains and summaries are usually expressed on a scale ranging from 0 to 100 where higher scores indicate better health for that domain. Simpler alternatives to the 36 questions have been developed by QualityMetric, SF-12®[52] and SF-8™[53]. The 8 domains are:
    1. Bodily pain
    2. Physical functioning
    3. Role limitations due to physical health
    4. General health
    5. Mental health
    6. Vitality
    7. Social functioning
    8. Role limitations due to emotional health.

Disease specific quality of life measures and hip specific outcome measures are separated by some into two entities [54]. As they are both limited in their extent, they are grouped together under disease specific measures in this thesis. There are currently 3 commonly used disease specific measures for hip osteoarthritis and THR:

  • Harris hip score: The score was originally introduced for evaluating acetabular fractures [55, s 69], but has since been widened to other hip related diseases [56], [57]. Harris hip score was originally not intended for self-reporting, but studies have shown that this is feasible [58]. The score consists of 10 items that are merged into a score ranging from 0 to 100 where higher scores indicate better hip function.
  • Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC): The WOMAC® score was developed in the 80s and consists of 24 items that are summarised into 3 subscales[59]–[61]. The subscores range from 0 to 100 where higher scores indicate poorer hip function. The 3 subscales are:
    1. Pain
    2. Stiffness
    3. Physical function
  • Oxford hip score: The score consists of 12 items add up to a score from 12 to 60 where higher scores indicate poorer hip function [62]. The scoring was updated by the original authors in 2007; the new score ranges from 0 to 48 and has flipped the scale, i.e. higher number indicates better hip function [63].

This is just a handful of the available disease specific hip scores [54], [64] and there is an ongoing effort in validating and translating new measures [65], [66]. PROMs are generally believed to be less susceptible to interviewer’s bias as the majority of them are self-reported, since patients answer the questionnaire without any aid from study personnel. The lack of supervision can increase the risk of misunderstood questions and skipped answers [67, s 500]. Recall bias can still occur and it is therefore important that the health states are prospectively recorded [68]. There is also a risk of response bias, i.e. that the patient responds to the survey differently from those non-responding. This stresses the importance of maximizing response rates [69]–[71].

PROMs are also believed to be more susceptible than other outcomes to cultural influence. Dieppe et al. reported large variations between and within countries in disease severity when deciding to operate [72]. This is something that I explored in one of the thesis articles.

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